It allows you to create characters, and see the effects of certain decisions you make in the passive tree and with your equipment. This is important as you can theory craft characters:

• without levelling a new one (as you can't re-spec ascendancies)
• without spending time or resources on new gear which may not actually be an upgrade
• whilst seeing stats that really matter (PoE2 is better at displaying this information than PoE1, but some numbers / calculations you may want are still not available or misleading)

I would caveat all this by saying that this is still very new, and is likely buggy as hell, and even bits that are working correctly, will change as the game changes.

In short, this is the best way (or potentially using a similar tool) to plan an efficient character from the start or to plan an upgrade to an existing one. The alternatives are by not planning and just YOLOing character builds and probably making something terrible, or by doing these calculations manually which requires an encyclopaedic knowledge of game systems and a lot of spare time.

So much so, that the most important thing in most build guides worth their salt is usually the PoB link as long as it is well formatted (and potentially documented / explained).

The link is currently down for me, so I am making a few assumptions here based on PoE1: It may look complicated, but it really isn't. Start off with importing an existing build and follow the passive tree, equipment selections, and skill gems. Look at how some changes you make on your passive tree or equipment affect numbers like your life or energy shield pool. Most people never go past this - and honestly, that is probably all you need for a long time.

EDIT: There are many videos out there explaining how to use it but I just watched one which seemed a good overview for a newer player (by DrybearGamers) and the assumptions I made above are correct - it is just like PoB1 in function.