[EricH]

There is a third reason to use bearing blocks. And a fourth that is connected to that third one.

3. Because if you don't drill the holes to the right size, the bearing can fall out. (big ouch) Or it doesn't go in, requiring a bigger drill, which increases the risk of oversizing the hole. Bearing blocks have the hole drilled for you (and generally speaking, right the first time).

4. In case of chain tensioning by sliding the bearing around, the blocks often have built-in tensioners. Oh, and did I mention that this sort of tensioning system generally needs some form of oversized hole/slot (see #3), so a bearing in a hole doesn't work as well?

*Note: #4 does not apply to "tensioning by sliding transmission", which is another possible method.