It's a good idea and probably should be done on a weekly basis if you use the database a lot. Basically, the more you use the database (and particularly if you regularly modify the structure -- queries, forms, reports, new table fields) the more "holes" it gets.
Compact and repair removes the holes without destroying data, and makes the database less prone to crashing. And you don't want a crash -- recovering a corrupted database is not pretty. By the way, I probably don't need to say it but back up regularly too.
Denis