There store recomends you some items like ''essential'' ''offensive'' ''defensive'' etc, the ''Essential'' however are the most important for each character right? the ones that you must buy firstly
helpfulolguys1 votesYeah... but not always. Like kalista 
with IE
and shiv
but always buy bork
and runaan
.
Bottorbn1 votesnot 100%, but highly recommended depending on the situation, and your comfortability with the champion
Critmaster Garen1 votesnot the first item you should buy, but often core item for the champion. just something that you should build at some stage of the game.
the best thing to do is to really read the tooltips of a champion and make note of how the champions abilities work and his scalings.
for example i used to buy ghostblade on garen, when nobody else would. it wasnt even recommended on him at the time and i would get shit for buying the item in the past. over time people have realized that its actually a good buy on him, and now its even a recommended item.
its most important, that you take a look at a champions kit and see what stats he scales well with. and then look which item provides you the most cost effective combination of those stats.
junglerboy161 votesNo, they are just the items that Riot recommends you buy.
When playing Varus I never use the recommended items because I play him AP.
When playing Master Yi I usually build the "essential" items, but they are rarely all the ones I rush first.
