If your iTunes library is too large to hold on your iPhone or you don't want to monopolized all the space of your device with music files, you can turn on the "Home Sharing" feature. This feature allows you to play music from your iTunes library from your iPhone. Just follow these steps:
Click the Advanced tab> Enable "Home Sharing"
Enter your "Apple ID" and "password" and click on "Create Home Sharing"
On your iPhone
Go to Settings> "Music "
Under "Home Sharing" enter your "Apple ID" and "password"
Play music from your iTunes library directly on your iPhone
Open Music application
Click the "More" button at the bottom right corner and select "Shared > library xxx"
Select your music track and tap to play
Note: Your PC and your iPhone must share the same WiFi network for this feature to work.