Why are there so many 2.5 SC transactions?

The host tracks your current IP address and automatically re-announces every time that the IP address changes. If you have manually set the host's net address (which can be IPv4, IPv6, or a full hostname like myhost.sia.tech), then the host will stop tracking your IP address and instead just use the one you have set manually.

Each host announcement puts a transaction on the blockchain, and incurs a 2.5 SC fee. These announcements are a requirement if you have a constantly changing NetAddress because they are the only way that renter's can discover your new address and download their data. To stop these announcements, you either need to get a static IP address (your ISP can help you with this), or you need a solution like DynDNS.