When you like an apartment if the "Search Apartments" section, you are indicating an interest in a specific apartments. This allows other users that are interested in the same place to connect with you and see if there is a mutual interest to check out the apartment and possibly join together on a lease. Also, any apartment that you like will show up on your profile under "Apartments I am considering" section. This will allow you to focus your search and connect with people with the same interest in apartments that you like.
By liking an apartment, you won't be solicited by any external company or individual. However, someone from Diggz may follow up with you to see if you need any help with the apartment search.