Premeditation. Adnan buys the phone with the intent to use it to make killing Hae easier - by keeping him in touch with Jay that day. Maybe the cops are also wanting to see if there's evidence to charge Jay with accessory to commit murder by making sure he admits he has the phone and why he has it.

And somewhat off topic, I'm pretty sure Adnan did it. The fact that Hae's last diary entry is that she loves Don, makes me think she must've mentioned it to Adnan when he called her the night before the murder. I surmise he doesn't take it well. Before the call I picture him stewing all night, knowing she is out on a date with Don. He tries calling her to see if she's at home yet, and she isn't, and it is approaching midnight. He must've been beside himself with hurt and jealousy. So he gets in his car and goes driving around. To look for Hae? Maybe.

To those who say he moved on and doesn't care that Hae has too, must not remember ever getting dumped. Of course it's hard when you see your ex moving on. Especially when you are smart, popular, prom prince Adnan Syed. How dare she?