There’s no technical reason for 320 CBR but the space loss from a VBR ~224 encode to the CBR 320 is minimal compared to 320 vs flac around 1000. I do keep two copies of my collection for space reasons though - one in flac and one at 320. My phone is still a limiting factor to from a space perspective, I can’t store my entire collection on it in flac but I do choose to use it where I’m actively listening to an album through the weeks. The rest can be mp3 because it’s usually a single song that gets in my head.