Dad’s short answer: It matters if you have furniture with low clearance, and not much otherwise. Lying flat lets the machine slide under a couch, a bed or a kick space instead of stopping at the edge. It's one of my favorite features on the machines that do it well.

What lay-flat actually buys you

Most floor washers stop at maybe a forty-five degree handle angle, which means the lowest thing they can get under is fairly high. A lay-flat machine drops until the handle is essentially parallel to the floor, so it slides under a couch, under a bed, and into the toe kick under cabinets.

In a house with a lot of low furniture that's a genuine difference, because those are exactly the spots that never get mopped and where dust and pet hair accumulate. If your furniture all sits on tall legs with plenty of clearance, you'll rarely use it.

Check the housing height too

Lying flat is only half of it. What actually determines whether the machine fits under your couch is the height of the body when it's flat, not the handle angle. Some machines lie flat and still have a tall housing that won't clear a low sofa.

Practical advice: measure the clearance under the lowest thing you care about before you buy, the same way I'd tell you to measure a doorway threshold before buying a robot. It's a two-minute job that saves a return.

Dad’s Take: Lay-flat is only useful if the body is short enough to follow the handle. Measure under your couch first.