As others have explained, TV providers will sometimes use the national feeds on their platform if there is no local affiliate within a market. As for the stations, the feeds still serve a great purpose. Stations will sometimes air a program as it is feeding via satellite in real-time, such as with "PBS News Hour" or "Frontline". Stations may also record programming from the feeds for later airing (focusing on HD01 and HD02).
The same services provided via satellite (except FNX) are also provided to stations via the IP-based interconnection system they use, called sIX ("six"). This has some benefits, such as one system acting as backup in case there are issues with the other. sIX is a cloud-based system and is the primary source for almost all programs that air on PBS stations, including programs from APT and NETA. The program feeds on HD01 and HD02 are sourced from the same files that are on the sIX interconnection system. sIX can also act as a backup for stations that record content from HD01 or HD02. If there is an issue with the program feed on HD01, there is no issue because the stations can just record the program from HD02 three hours later. However, if an issue impacts the program feed on HD02, stations will need to download the program from sIX. It's a lot more convenient to source programming from sIX than from recording a linear satellite feed, yes, but I do miss recording pre-feeds from HD03 (and the former HD04 and HD05 feeds).