While networks generally wait until after a new episode airs to offer it to viewers through other platforms, there are exceptions, such as NBC's decision to release the entire season of "Aquarius" on Hulu, NBC.com and other platforms immediately after the first episode aired.
Additionally, sometimes networks release the first episode of a series online in order to create buzz around the show before it officially premieres. For example, Syfy made the first episode of "The Expanse" available online several weeks before it premiered.
Sometimes individuals leak episodes of a popular show prior to their air date without the permission of the network that owns the rights to the show. This happened in April 2015, when the first four episodes of the fifth season of "Game of Thrones" were leaked to BitTorrent without HBO's permission, including three episodes that had not yet aired.