Personally i find having goal gives character more texture to his own story and background, allowing to grow him forward in easier and more substantial way.
But that's not to say how such thing is necessary, and to presume how goal is required to justify someone existence would be big understatement to make.
Because there are various other desires characters may have going for them beside only their personal main dreams, still being vital to story.
For example Max didn't had goal but he was still valuable asset to team helping cast to navigate way toward next gyms or help them when they got lost, gave advices about gym leaders and battle strategies as well bailing group out from sometime dangerous situations. Brock aside from breeder dream wanted to travel and see more of world , get chance to experience new places and pokemon from first hand and enrich his roaster with new captures.
Same applies to Misty who aside from pursuing water career wanted to explore new regions, have fun with friends by learning more about various types of pokemon, new innovative ways of battling and run away from sisters shadow wanting to make name for herself.
So its not like goal is the only thing which defines character and fills him with purpose, with this show being more about journey rather than destination.
But anyhow so far we had two definition of goals in pokemon. Ones being clearly explained and defined like goal of becoming top coordinator knowing what kind of competitions it involve and whats needed to do to become one(such as winning GF).
Along with more unclear ones such as becoming dragon or water master, S class connoisseur etc which defined what they mean explaining each term abit, but lacked proper explanation and visibly defined way through which you can build up and accomplish such thing.
However ill be fair in here. If there is one positive aspect unexplored careers have in certain situations is ability to bring that dash of mystery to it allowing writers to take it through several fresh directions making them more unpredictable/exciting in sense of not knowing what to expect next. Keeping you in suspension. As opposite to formulaic paradigm fully explored goals has taking away from originality and excitement due to being set up what structure and procedure needs to be followed.
Each goal has its pros and cons depending how you look at it.
p.s. Regarding Misty water master career im not sure why people act like its unknown what this goal means.
It was established in original series how Misty goal involves becoming strongest water trainer in world being on pair with E4 members. Who are best mono trainers often being labeled as one type masters, with Misty showing clear fascination over Lorelei being her idol. Also judging by wording from sea priestess Maya title of water master means becoming water pokemon expert which has power to understand sea creatures and use them to full potential,being recognized and respected among all water trainers worldwide. For same reason Whirl Cup was introduced as one of such competitions where you can gain more experience,make step forward and prove yourself to others being directly linked with water master thing serving as one of benchmarks through which you come closer toward such title.
In addition it was established with Cilan connoisseur dream how there exist rankings for those which identify compatibility between trainer and his pokemon. C being lowest, while S highest class connoisseur can achieve making you top expert in fiekd of determining accordance and level of harmony there can exist between humans and their pokemon partners.
As such we also know judging by Cabernet history how there exists school where you can become certified connoisseur and likely special tasks /tests you need to pass getting estimated by other top class craftsman in this field to determine if you are good enough to advance on next level.
In nutshell we do know what Misty water master goal or Cilan S class connoisseurs dreams mean, being established aim they are looking for. We only don't know full picture of procedure, path you need to follow to accomplish this dreams.
Sam applies to Brock breeding career.
Arguably Misty had slightly better defined goal than rest of crew with vague goals, because with water tournament it was set up how there exists specific tasks which serve as way to rank up as trainer.
Not making them any more vague imo than Ash never ending pokemon master quest still not knowing what it involves aside from winning league and battling E4. Iris dragon master goal which lack proper explanation, clean-cut direction she needs to follow to earn right to gain title of master. Etc, etc.
I think we're looking more for an answer of what specifically it takes to become a Water Pokemon Master. The steps toward it, not assumptions. Things that were stated that must be done to achievement that goal. It was really vague, then again it was OS.
During Whirl islands it was clearly stated how winner of tourney gets title "Alpha Omega of water types"being stated by prof. Elm and later sea priestess Maya how "water heroes"(in japanese convoluted form of water master) in past were called by that name, being obviously one of steps someone needs to enter and fulfill to gain that tile. Wining one tournament wont make you master, but it is one of steps which brings you closer, so entering water tournaments like Whirl Cup showed to be directly connected with Misty dream.
