Blocking a pattern helps set it.
When you finish a project, it has all the oils and dirt from your hands (even when they are clean), so washing or soaking it is always a good thing.
Wool will always go back to the shape it first dried in. So...if a project drys in a crumpled heap, it will remember the crumpled heap shape every time it dries after being washed.
Blocking it involves laying (and sometimes pinning) the project out in its intended shape. It will dry flat and remember that flat nice shape the next time it dries.
With lace projects, blocking makes a significant difference. It opens up the eyelets and makes the project come alive.
It doesn't hurt to block acrylic projects either. Acrylic was advertised once to not have to be blocked, but it won't hurt the project to give it the last step of blocking. I believe there have been debates here on KH about the need or not to block acrylic.
Hope that helps