It's something you don't have to do but it can give you a clear advantage if you're good at it. Tends to impact your skill level a fair bit. Same with lesser known mechanics like stacking and pulling. You only need to at higher skills levels. Personally I like it and the way it give a few more options, especially when making snap decisions ("should I starve them of xp or get that bit of gold to buy X from the side shop?"). Also lets you use some interesting skills like Lich's sacrifice ability to eat your own creeps for mana/denial, or Doom's to absorb their abilities.