Crustle is an archetypal rock type. Low Speed and Special, high DEF and ATK. Which means it is in an already crowded field. Crustle's bug typing gives it some differential from other Rock types barring the passive Shuckle. Unfortunately this typing is more of a curse than a blessing, providing a weakness in Rock moves and losing resistances in Fire and Flying moves in exchange for losing weaknesses in Grass, Fighting and Ground moves.
So what sets Crustle apart? Its moveset. Having access to both Spikes and Stealth Rock is a good start, and no Pokémon is going to turn down Knock Off and Shell Smash as options. Crustle may be outclassed at most of what it does, but in the lower tiers, you have an unmatched support option.
Treacherous Ground
Crustle
Ability: Sturdy
Item: Mental Herb
-Spikes
-Stealth Rock
-Knock Off
-Stone Edge/Rock Blast
Nature: Adamant (+ATK, -SATK), Calm (+SDEF, -SATK) or Jolly (+SPE, -SATK)
EVs: 252 HP/252 ATK/4 SDEF OR 252 ATK/4 SDEF/252 SPE
Crustle has a series of factors that work great as a entry hazard setter. Sturdy acts like a built in Focus Sash, freeing up Crustle's Item slot for other items. Spikes and Stealth Rock are a given, and advantageous to have them one Pokémon. STAB rock moves provide an excellent deterrent vs most Defog users, with Stone Edge for more reliability, while Rock Blast can break through opposing Focus Sash leads. Knock Off is the last move, providing additional coverage and further team support by removing items.
Item is Mental Herb, to prevent disruption via Taunt. But this leads to consideration of your EVs and Nature. Do you want to maximize bulk so Crustle could possibly come back later in the match? Or do you maximize speed, so you can outspeed opposing Taunt users and get a 2nd entry hazard up?