Preamp - what's the purpose?

a preamp has MANY benefits.

Input source selection.
Volume and balance controls.
Tone controls, YES, tone controls. They are invaluable in my opinion, especially if the source music is not perfect, also room layout, speaker placement, so much can affect the sound and tone controls can go a long way to 'compensate' for any short comings.
I use Baxandall tone controls in my custom builds, one the best tone control circuits out there. I also offer a tone defeat switch, very handy when a 'pure' line preamp is desired for certain listening sessions.

@heartwoodcorp

glad to hear that you are enjoying your Aretha!

Many preamps have subwoofer circuitry with high pass filters and gain control too. The switching ability, volume control, remote control and multiple analog and digital inputs/outputs are very important to me.  The pleasure of a tube preamp allows for a different dimension in sound quality too.  

That’s the thing these days, most dac sources have subwoofer circuitry with high pass filters and gain control.  My Oppo 205 has all these features plus many more including surround 7.1. I run it direct to my power amp through balanced XLR connection.

Some people have multiple sources and need a switch.  I have 2 inputs but have an amp with 2 inputs.  My new amp I ordered custom has 3 inputs, one for a spare.  

Some DACs don't have volume control, so they use the pre for that.

Some people think a preamp improves the quality of the sound.  I say it can only color it.  

And I do it just like you--DAC into my tube amp.  

Jerry