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.