There is much thought and time put into a character in Path of Exile, from the progression to the final build. Due to the level of depth and flexibility afforded by the game, there is a whole lot of freedom to be had in a character's development. That means you don't have to follow a character build guide down to the very letter as the game gives you the chance to explore other options and come up with your own variation of a pre-existing build.
This is something that the developers at Grinding Gear Games intended for their game to encourage, and that's something you should take advantage of as a Path of Exile player. With that said, you enter the wonderful and marvelously complex world of theorycrafting in Path of Exile, which is like a whole new can of worms popped open right in front of your eyes. Whenever you pull up that intimidating yet beautiful passive skill tree in the game, you're opening a Pandora's box full of possibilities that theorycrafting will enable you to make the most of.
This is basically a more in-depth character build planning that takes most major aspects of the character into consideration. It's basically the process of fine-tuning a build for the first time based on game knowledge and research at hand, including the Path of Exile items that either enhance or form the basis of the build. You must plan ahead what class you are to take, what Ascendancy class you're aiming for, as well as the offensive and defensive plans that can be relied upon in combat.
The bulk of the offensive and defensive plans are stat benchmarks that will ensure the character's overall effectiveness. Being able to reach a minimum attack damage and speed or spell damage and casting speed will ensure that you can deliver adequate damage to kill all kinds of enemies consistently. Having a certain percentage of armor, resistances, increased health and regeneration, and so on must be done to guarantee survivability so you can stay alive even in the most intense of fights in the field. You must consider the POE items and skills that go into this build for those benchmarks.
A big part of those considerations is determining what kind of enemies you're going to fight and in what conditions. If you wish to solo a particular boss, then you should plan for that boss. Coming up with purpose-oriented builds like a character for killing a certain boss can be quite a fun challenge. You must also consider what room you have left for growth and further development. Perhaps you can add in some more special gear to make the character stronger if the opportunity presents itself.
Theorycrafting is indeed for those who want to take the game a bit more seriously, and it can be a daunting and tedious task for those who are unprepared for its intricacy and sometimes mind-numbing obsession to detail. However, that's exactly the kind of thing that a game like Path of Exile paves the way to, so it's something that a serious player of the game should be able to get used to because this game is indeed all about the details.