How important is a preamp for purely digital sources?

You need to tell us what your alternative is - **something** must do switching between sources (unless you are doing all that in the PC/whatever) and control volume. You may, or may not, requires a small amount of gain - its systems dependent (room, speaker efficiency, DAC drive level).
Digital volume control in any PC/MAC/whatever has serious issues that i wont get into here.  And they still exist.  Volume control within a DAC can be very good, or very bad.  In order to perform digital volume control in the DAC; it must operate on a roughly 30 bit or better native signal - some chips do this, others don't. ESS has a nice preso online explaining this and showing the numbers (which are scary). Roon's digital volume control is quite good, but even they admit its not on par with fixed volume. So its a trade-off.
http://esstech.com/files/3014/4095/4308/digital-vs-analog-volume-control.pdf

I'll add one more comment since you used the wording "how much benefit?".  Components dont improve things.  A truly great components simply doesn't degrade the signal..... much. So you are really trading off the evils of the digital volume path vs the evils of an analog volume and gain path.  Conductive plastic vs digital manipulation and loss of resolution.
To give you an idea of the magnitude - were you to use the volume control in a mac or PC, you could easily get below 11 bits resolution, while audiophiles argue whether 16 or 24 are needed.  Holy orders of magnitude Batman!

If your solution does a pretty good to terrific job of digital volume control, and you don't have a really good DAC, I'd probably go DAC by the way - but the devil's int he details!
G

A preamplifier vs direct to amp to dac ,a good preamp always better especially a vacuum tube preamp

Any stage can only distort. You may liek the euphonic distortion of a tube preamp though, which is valid

t allowsbettrr dynamics
soundstage depth as well as image depth .good power supplies
and transformers give you the amplified dynamics ,

You cannot add dynamics (not without DBX like expansion). So this is false. maybe for some reason the poster gets the subjective impression of dynamics - louder maybe. We all know the Fletcher-muson effect. (e.g.: the loudness button!)

preamplifier it takes the signal and amplifies the signal.

Well, yea, superficially, but 99% of the time a preamp is actually delivering fractional gain (its only attenuating the signal).  If this were nto true you would nto have the option of driving the amp with the DAC at all!
obviously, if the DAC does nto have a high enough output level, you need a preamp.  I presume the OP knows otherwise.
What a preamp CAN do is fix impedance matching issues. if the DAC has a hgih impedance output, and the amp a low impedance input, you Will lose dynamics and a preamp might fix this. With well designed equipment this ought to be rare.
G