Roast beef is rich, salty and often served cold in sandwiches. It needs acid and crunch to stay interesting. Sweet brown pickle is the classic choice because it brings tang and a bit of fruitiness that balances beef.
Pickled onions also work brilliantly, especially with mustard. For a crunchier finish, add gherkins or pickled cauliflower. If you like heat, chilli piccalilli adds mustard tang and warmth.
The goal is balance. Beef, butter and bread can be heavy. A good pickle makes it feel fresh again.