Kitchen sinks see food scraps, coffee grounds, vegetable bits and the odd piece of cutlery trying to escape. Bathroom drains get hair, soap residue and tiny items. Without a barrier, all of that heads straight into the pipes, where it gradually builds up into stubborn clogs.
A simple mesh or perforated strainer acts like a gatekeeper. It catches solids while letting water flow through. Emptying the strainer into the bin regularly is far less trouble than calling a plumber to clear a blocked pipe or dealing with water backing up into the sink or shower.
In kitchens, it also helps keep pests away, since less organic matter sits inside the pipes rotting quietly.
It’s one of those “do a little now to avoid a lot later” tools.