What guards actually do
Good guards keep the big stuff (whole leaves, twigs) out of the trough. Fine debris still gets through or sits on top: oak tassels, pine needles, shingle grit, seed pods. Instead of cleaning the trough two or three times a year, you are brushing off the guards and checking the seams once a year or so. That is a real improvement. It is not zero.
The types, honestly ranked
Fine stainless micro-mesh is the serious option and the priciest. Solid surface-tension hoods work but can overshoot in hard valley rain. Cheap foam inserts and bottle-brush guards clog themselves and are mostly a way to make cleaning harder later. If a quote does not name the type, it is not a quote.
When they are worth it
Heavy oak or pine canopy over a two-story roof is the strongest case, since each avoided cleaning is expensive and the ladder work is real. A single-story ranch with one modest tree probably never pays back a four-figure guard install. Run the math on cleanings avoided, not on fear.