Some people are great at rhymes and not great at making beats to go with them. Pretty much every rhyme has a cadence it should be spoken to. This sets the rhythm for the rhyme. It is the same in poetry of different forms, and rap is a form of poetry. Rap just gets a beat to go with it to make it a song. I can write rhymes pretty well, and I buy hip hop beats to merge with my rhymes. There are so many available, and they are separated even by the mood of music that you want to buy. It takes a lot of searching and listening to track samples, but I can find great beats to go with my rhymes.
I see it as a collaboration. I write the lyrics and someone else writes the music. This is as about as common a thing as you can get in the music industry. There are many songs that are hits that have separate lyricists and creators of the music. Ideally, it would be great to be able to collaborate on the fly with a beats artist to be able to tweak a track to fit the actual flow of the rhyme. However, I have found that there are so many beats out there that are available that i can always find the perfect sound to go with my words.
Maybe one day I will learn how to make my own beats. Until then, I will buy hip hop beats for my rhymes. It works for me. Buying them under specific contracts frees me to use them as a I please too. This way there is no legal battles to worry about if a song makes me a lot of money. It is important to know what licensing you are getting when you buy beats.