Pre roll ads are how stations monetize their streams, especially to listeners that are not local and can't be counted when trying to sell to local businesses.
It costs $$$$ to stream, and people connecting from far away that can't be monetized are a drain on the stations bottom line.
That is why in the midst of Covid 19, many stations geo-fenced so only those within the listening area could connect, or they dropped the streams all together.
One of the Moderators on my FB radio forum said he was able to save a full time on air position with the money he saved by geo-fencing his stream.
A good station that develops a reputation for playing good music ( in my case it was WAKY) attracting many out of area listeners could easily be paying royalties in excess of $700 or more a week and getting nothing back in return.