Above is a stolon from a creeping bentgrass plant I pulled off of #1 fairway. Bentgrass has two methods of reproducing:
1. Seed
2. Vegatatively through its stoloniferous growth habit.
Stolons are above ground stems that produce new shoots of grass. When you take a divot and fill the void with sand, it often recovers and heals by way of stolons and its creeping growth habit. Warm season grasses like bermudagrass growth in a similar fashion, only much more aggressively. The also contain rhizomes, or underground stems. This is why you can new get rid of Bermudagrass; some of these rhizomes can grow as deep as 12" or more!
Other grasses like perennial ryegrass or poa annua have bunch type growth habits. There only method of reproduction is seed. This is why we aerify and interseed ryegrass (or tall fescue) lawns to increase density.
Creeping grasses like bentgrass will certainly benefit from interseeding, particularly if you are introducing a new variety, but we use tools like fertility, vertical mowing (grooming) and growth regulators to achieve greater density. Think of a stolon like another plants you are familiar with. If I cut that stolon, buds will break creating more individual turf plants. Hence, the density increase.