Why do I have to redownload music I have already purchased?

HELP PLEASE!

I am finding it increasingly frustrating that I am not able to listen to music I have purchased. My albums are showing up split on my music app in random orders and some songs, yet again are randomly missing showing the iCloud symbol. I purchase music for the intention of listening to it then and there not for iTunes to decide what I can and cannot listen too. If I have no internet, I cannot listen to the song.

Please help.

Many thanks