Preamp - what's the purpose?

I have tried it both ways- in my case a PS Audio Direct Stream DAC to an Audio Research SP 15, an Audio Research Ref 6SE and direct to amp.

My experience is this: If I don't mind having lower volume recordings preventing me from getting the DBs I want (I like 80-96DB) DAC to amp is just fine.

Alternatively my SP 15 added a certain brightness to the mid/highs which I wasn't satisfied with. 

My Ref 6 SE was a big volume booster- and nearly neutral. Zero added shades. It did add discernable separation of string instruments (airness?) and the feeling I could "see" behind the cello as it played. I thought that was really cool and worth the price of admission. 

I no longer use phono and CD so switching between sources was not important to me. 

There is also the budget issue. DAC to amp saves you the costs of buying and maintaining a preamp. At least for me having great music and a small budget when I got back into the hobby made DAC to amp a great deal. 

I hope my "eye witness" experience helps you with your decision. 

Functionally: Volume Control is the most common need. Thus a Preamp is an attenuator, most combined with a gain stage. Passive is a switcher, attenuator with no gain stage, perhaps other features.

Pre-Amp, (the pre) (ignoring features, switching, tone/filter/modes/etc) are needed to pre-amplify low level signals up to Line Level, then off to the amp., i.e. Moving Magnet Cartridge’s low signal strength

Any Line Level Strength signal can go straight to the amp, BUT, where is the volume control?

’Back in the Day’: Amps were/are designed to receive ’line level’. Original Sources, i.e. Mono FM produced/sent 1 volt to an amp.

Stereo: you need another amp, stereo source like Stereo FM. Now two 1 volt out to two amps to two speakers.

Vinyl: original ceramic cartridges were strong signal, (remember RIAA EQ had to occur somewhere), originally within the preamp, now often a separate component: then Line Level Signal to Amp. MM cartridge signal strength, stronger than Ceramic, into MM Phono for RIAA EQ, and boosted to line level.

Pre-Pre Amp. MC cartridges, lower signal strength, in order to use existing MM RIAA circuits, needed to be pre-amplified up to MM signal strength, then to MM Phono EQ input, then out at line level. SUTs, i.e. Step Up Transformers are pre-preamp solutions.

Modern sources, i.e. CD Players, digital output ... can send much more than the old 1 volt line level.

Some Preamps, like my current and past McIntosh preamps, and prior Tandberg Receiver, let you ’level’ the various signals to each other, so there is no need to alter the volume control when switching sources, i.e. Phono 1; Phono Two; Tape: matched level controls, then off to the Primary Attenuator.

I have been using a Decware amp fed by a DAC and phono preamp for more than a year.  I decided a remote control was needed and ordered an Erhard Audio Aretha preamp.   It made my system sound better.  Soundstage was the largest benefit.  Instant gratification!

A preamp is primarily a switching center. If you don't need it don't worry about it. 

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.