It allows for some fiddling around with your alignment as the biscuit joiner will oversize the slot for the biscuit a bit allowing you to line up the joint. Ok-I can’t help myself: Biscuit joints vs. Doweling quickly summarized: Biscuits: More play, faster, more expensive tools, better pull-apart strength. I’m merely a carpenter, and there won’t be much-if any stress on my joints so a biscuit will work just fine.
Years ago, back in the 1990s and early 2000s, a biscuit joiner became a very popular woodworking tool. Well, as much as I admire Norm Abram, a biscuit joiner may be one of the most useless power tools you can own. If you’re new to woodworking, or if you’re a maker, you may not even know what a biscuit joiner is.