No reason. Apart from:
- XP
- Merits
- Influence, Salvage, Incarnate components if you're 50, and extra Notices if it's a WST
- Camaraderie and banter with other players
- Enjoyment of actually playing the game and doing content
But these are reasons often overlooked by players whose only goal is the constant strive to always be advancing their character to the next step on an arbitrary ladder. Grind for more components. Maximize further progression! Get T4 in every slot! Must maximize numbers. Fun? What does that profit me?