can someone please explain what this mean "sets mode: +l 86" like why do bots do this? whats the point of settin limit?

It's normally so that if someone was flooding (or trying to) the channel, it would help stop the flood, eg:

Someone tries to send 20 clones into a channel, but because the limit only allows 6 more people to join, only 6 of the bots will get in, thus reducing the effect of the flood.

Simply put from the mIRC.hlp file:

Limit to N users
This limits the number of people who can be on the channel at any one time to N people.

l <number> channel is limited, <number> users allowed max