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.
The national-brand markup
The heavily advertised names charge a large premium for a fairly ordinary micro-mesh, and the price famously drops the longer you hesitate in your living room. That is a sales model, not a product advantage. A capable local installer can fit comparable or identical mesh for a fraction of the in-home-demo price. If you decide guards are worth it, get them from someone who quotes a flat number and leaves the pressure at the door.
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.
Adding guards with new gutters
If you are already replacing your gutters, adding guards at the same time can be efficient, since the installer is up there anyway and can size the guard to the new profile. Just price it as a separate line so you can judge it on its own merits, and do not let "while we are here" talk you into the premium product. The math is still cleanings avoided, not fear.