There are two mode in LESS compiltation:
Client-side compilation : for not production mode, using the native
Server compilation : just for the production mode.
You can set that in admin :
Stores > Configuration > Advanced > Developer > Store View > Front-end development workflow.
To see the changes, you have to remove the
pub/static content. and by doing this so many times, and every time you make a change, it becomes annoying and not necessarily practical. so grunt is a good thing for that.instead of clearing all this manually, grunt will do it for you, you just have to configure it properly. More informations.
I wrote a post with the complete configuration and with examples.
hope it helps you.