If, when you clean a paint brush, you don't bother to get all
the paint out of the bristles, the paint will dry in the brush and stiffen it. Yes, it can be tedious to get the last little bits out of the bristles near the ferrule
, but think about it how much paint will accumulate if you leave as little as shown in the photo here every time you half-heartedly clean a brush.
If you're using cheap brushes and can afford to replace them regularly, then you certainly don't need to bother with cleaning all the paint out of a brush. But if you're on a budget, or use expensive brushes, then it's worth the time spent.