One of the benefits of subscribing to Spotify Premium plan is the ability to enjoy higher quality music streaming. However you have to manually adjust this streaming quality settings.
Go to “Your Library” tab and tap on the Settings icon on top right. Look for “Streaming Quality” and tap to switch to Extreme which is the highest quality at 320kbit/s. You might also want to adjust the “Download quality” settings on the same page to Extreme.
Please note that the better streaming quality imposes higher Internet bandwidth. Do watch out for your usage if you’re under limited Internet plan.