If you are playing riven who generally goes top lane and starts off with a Doran’s blade, in the jungle then you cannot start with a Doran’s blade.
You will need to get a completely different build route and will need to get a hunters machete in order to survive the tough jungle monsters and the clear that it has.
Likewise if you are playing Annie a champion that usually goes into the middle lane and you play her support you will need to build a support item such as a Sightstone in order to succeed in the lane.
Supports can usually go mid and mid laners can also go support. Items builds are a key piece and one of the most important if not the most important thing in the game.