Most watches made in the last 60 years or so have some form of seals to keep out dust and moisture.
The first picture shows a Rolex case with its seals removed. One for the case back, and three smaller ones for its so-called Triplock crown.
Even if one intends to never put a watch in the water, wherever possible a watch should still have seal replacement, as there is water and oxygen in the air around us, which over time will leak in and cause corrosion/oxidisation of the parts inside, the hands, dial, and movement.
After new seals are installed, the watch is tested in a pressure test machine. I use the same top-of-the-range Witschi machine that is used by most watch companies. It gives both a negative pressure test and a positive pressure test, and is accurate down to an inrcedible 1μm or 1 millionth of a metre.
See the mirror-shine on these Breitling hands? this shows it has suffered no significant leaks in the past. Regular seal changes and checks will keep it looking like this over the long term.
The next picture shows new seals being installed in a watch’s crown and pusher buttons. These new seals are soft and round. As they age they go hard and eventually fail, no longer keeping out moisture nor air.
Over time the parts inside the watch will become oxidised as shown in the hands and dial on this Omega.
Watch manufacturer websites typically advise that one changes seals every year, or at least every two years if one is intentionally submerging a watch.
If not intentionally submerging then one can be more lax, and in this case I recommend seals at 8 years.