I am not an audio engineer but from what I understand, increasing the resolution enabled the player to take more detail from the audio track and turn it to sound. It's like increasing the bandwidth on an Internet connection. If you have a large file to download and limited time to do it, the only way to get the file is to increase the size of your pipe (connection speed). That's the same with audio. If your phone supports high resolution output, leave it enabled. The cost in terms of resources (RAM and CPU cycles) is negligible and you'll get better sound for most of your tracks. The easiest files to test this are lossless tracks and higher quality lossy tracks like your 320kbps tracks.
Anybody correct me if I'm wrong. The lesson in how audio works will be much welcome.