In addition to what TerminalEncounter said, just more simply, it's a major part of DNA. No phosphorus, no DNA. Here's some general info on its importance to life.

PO bonds are super useful for biochemistry. If you bond a bunch of phosphate groups together it's like storing energy in a battery or spring or whatever - earth life (feels weird to quantify it this way but whatever) uses ATP to store energy but any alien biochemistry is probably gonna use it too, well maybe any alien biochemistry that is in liquid water. Dunno about those more wacky liquid methane or whatever ones they were speculating could live on Titan.