My reasons.

-You can use reusable water bottles forever.

-When you leave a plastic bottle in the sun plastic particles from that plastic bottle dissolve into your water!

-If you buy a plastic water bottle for $1 for 100 days that would cost you $100. But if you buy a $20 reusable bottle and it last 100 days you would save $80!

These are some reasons I would use a reusable water bottle insted of a plasic one.