Proxies become useful when you're botting on many accounts.
For example, when botting one or even a few accounts, you might not even need a proxy (i.e, You could just use your home IP).
If you're botting many accounts (More than a few), it would probably be advantageous to use proxies in order to separate the accounts, ideally individually (Each account with its own IP) or in groups (A few accounts per IP). This would greatly reduce chain bans and suspicion in the eyes of Jagex.
To add proxies, just open the DreamBot launcher and you'll get options to add the proxies from there.
P.S. If you are planning to use proxies, I would advise to use 'Residential' proxies instead of 'Datacenter' proxies.