Is there any reason to change my vocation?

Yes, there are two major reasons:

Any character level you gain will grant you increased stats according to your current vocation, which means a lvl20 character that has been playing as a warrior from level one will have significantly different stats than one that was a mage from level 1-19. While a lot of this difference can be mitigated with proper gear for your current vocation, it does affect the difficulty of combat since enemies stats do not scale with yours.

Secondly, and more importantly, any augments you unlock in a vocation is available to your character permantently, for all vocations. Some augments from the strider vocation are very useful for a fighter, and some fighter augments are even useful for a mage (like increased carrying capacity).

Both of these also apply to your main pawn, although augments are unlocked separately for you and the pawn, it doesn't automatically have access to your augments.

Edit: I might add that any non-vocation weapon skills are retained when you switch vocation. This means a strider with all dagger skills unlocked that switches to assassin will still have access to any dagger skills that aren't specifically marked as strider only (easily identified from the small class icon in the description). If you switch back to strider later, you'll regain access to the strider skills (i.e. you won't need to purchase them again). This goes for the vocations too, if you spend the exp to unlock vocations you can switch between them without any cost afterwards.