Since filing a trade-mark registration costs more at the outset than NOT filing it, and trade-mark rights really begin with use of the trade-mark in commerce rather than the filing, there has to be a good reason.
In fact, I can name six!
1. Prima facie evidence of ownership
2. Public notice of rights (on the Register)
3. Registered trade-marks are a clearly identifiable asset in licenses and purchase agreements.
4. More ways to attack infringers, passers off, and other disreputable parties.
5. Looks shinier / more professional
6. You can use the registered mark as a priority or foreign registration/use basis in other jurisdictions.
Maybe you have other ideas; please comment if you do…