A DAW is a computer program, and a computer only has so many jacks for audio inputs, and so many ports for USB inputs. Depending on the number of instruments you need to record at the same time, you might need or want to use a mixer to combine and balance the audio from a number of different sources before sending it to the computer to be recorded in a DAW. And the better the signal going into the DAW, the less you'll need to work on cleaning it up once you get it in the DAW. If you're just going to record yourself playing a keyboard, you probably won't need a mixer, although you might want to get an audio interface. But if you're going to record yourself playing keyboard while someone sings, someone plays guitar, some plays bass, and someone plays drums, then you might want to get a mixer. And even if it's just you playing keyboard, you still might want to get a mixer if you've got several different keyboards that you'll be using at the same time.