The thing about tabletop games is that it's not just about the games themselves.
It doesn't matter which format you play there will always be accessories that you can buy to improve, complement and protect your games.
For Roleplaying Games the perennial accessory is more dice. You can never have enough dice. With dice comes methods of storing them and there is a variety of dice bags out there whether it be the plain velvet to being adorned with logos to custom dice bags made to order from websites like Etsy.
When it comes to any game that uses cards the accessories available are significant.
At the very least you're talking about card sleeves from manufacturers like UltraPro, FFG and MAX Protection.
If we're talking about CCGs then the selection expands into deck boxes, playmats, folios to display rare cards for trading and much, much more.
If you're a wargamer then the obvious accessories are paint, brushes, glue and basing accessories. Companies like Army Painter focus entirely on these accessories whilst the games manufacturers like Games Workshop and Privateer Press have their own ranges of paints and other accessories. For transporting your figures you'll also look at cases from GW and specialists like KR Multicase and Battlefoam.
The variety of accessories out there is much wider than what I've put here. What did I miss?