it depends on your league's settings. it reads like (it says you change every monday) like it is a weekly line-up league. there are leagues that you can set daily. i'd bet somewhere on your team page there is a link to the settings, i'd look for that.
i'm not familiar w/ nba.com leagues or if you can even make your own settings over there. most leagues on yahoo are daily changes. the default on yahoo is daily, you'd have to create a league (or join a custom league) & change the settings for a weekly change league.
the advantages of a weekly change league is.... well i don't know of a good reason other than it's better for casual fans.
the advantage of daily change is just that, you can still go in & set a line-up for the whole week w/out having to come on everyday BUT, you can play guys that have games one day & bench them for someone else on days they are off.
another big advantage of daily changes is INJURY. if you are know that D Wade hurt his ankle on tuesday, you can take him out & put someone in you KNOW will play on wednesday. if Kobe has been in court all week but will be back for the weekend games, you won't miss out on what he does for the final couple games of the week.