From personal experience... the plastic mesh simply filters the size
of the debris so that what ends up clogging the gutter has much smaller
particles than it otherwise would. But the gutter is just as clogged.

The gutter guards that work are the ones that look like they won't. I
had "Gutter Helmets" installed on my prior house, and they worked
dandy. (In New Jersey, where we got some heavy downpours.)

The reason they look like they won't work is that they are metal
extrusions that cover the top of the gutter, then form a sort of "J,"
the apex of which reaches out over the edge of the gutter.

Now you'd think what would happen is that the water would just be
directed away from the gutter.

Not, so, Mr. Newton.

What does happen is that the water actually clings to the guard,
makes a U-turn over the edge of the guard, and goes in the gutter.

Debris, however, doesn't make the hair-pin turn, and ends up on
the ground.

Yes, it works.

Yes, it works in a down-pour.

The older ones, it's a bitch to get them off if you do have to clean
the gutters, or do work. Some of the newer models or alternate brands
which operate on the same principle have ways around this.

So yeah, they work pretty much as advertised. At least the Gutter
Helmets do. If I didn't live in a condo now, and had a house where
I was responsible for cleaning the gutters, especially gutters more
than ten feet up, I'd buy them again without hesitation.