Gain stages on Preamp

The main benefit of adjusting gain at the pre-amp it to match the amp(input sensitivity) and speaker SPL. Too much gain and/or too much amp sensitivity and/or speaker SPL and you will raise the noise floor substantially and reduce the range of the volume attenuation control on the pre-amp, i.e. the usable level of the volume control is between 8 and 9 o'clock if the gain is too high or the amp/speaker is too efficient. You loose small gradations of volume level which is usually available when your volume control is about 12 o'clock (+/-) on many but not all attenuators.

It doesn't make any difference whether is ss or tube, but the noise floor issue is much more common with tubed units and very efficient speakers (90 DB+) With speakers over 100db it can be critical.

Volume controls are very imperfect beasts. They are at their most linear completely open, out of the circuit. The further you turn them down the less linear they become. So the purpose of adjustable gain is to be able to use the volume control over the maximum part of its travel to reduce non-linearities and channel imbalance.

good answers by Newbee & Viridian. I'll add my 2 cents:
the gain control on a preamp is really an attenuator as already indicated. The CDP or any source outputs a fairly large signal that, if unattenuated, would exceed the max input voltage level of the power amp that is required by the power amp to produce its max output power. IOW, if the source signal is unattenuated you would be listening to music at max output power which would make your ears bleed.
So, you turn the volume control to adjust how much of the source signal gets thru to the power amp.
In many vintage preamps & integrateds you could see the 6 o'clock position labeled as 'minus infinity' i.e. max attenuation i.e. none of the source signal is getting thru.
Tube vs SS: generally, you are dealing with more noise in a tube cot but not always. There are some very, very good tube preamps out there that have practically no noise at low levels of attenuation.
Given that tubes can be as good as SS (or SS can be as good as tubes - depends on which camp you prefer to be in.... ;-) ), implementation of the volume control is more important. Resistor ladder vs. transformer type - some people prefer one type vs. the other. Many feel that transformer type volume control is more transparent than resistor type altho' Placette Audio made a very transparent resistor ladder volume control, if memory serves me correctly. Volume control can also be done digitally (as was the case in many Wadia CD players) as is the case in many digital power amps today & several DACs as well. Here, as the volume is turned down the SQ degrades rather rapidly. I'm sure that many strides have been made to improve this today but I have not had the chance to hear a present-day digital volume control.
Pros & Cons to turning it up/down: turn it down & your wanted signal is not much higher than the noise floor of the electronics used to create the preamp function. This means that the power amp is outputting a low wattage signal to the speaker & you have low SPLs in your listening room. Many speakers do not fare well at all with low SPLs in that the music does not have the vibrancy that engages the listener emotionally. There voltage drive to the drivers is too low for adequate SPL.
Turning it up too much, while you are operating in the more linear region of the volume control, you boost the signal & the noise (from the electronics). While your signal-to-noise ratio is high (which is what you want), your noise is higher too. Plus, with such a high signal level running into your power amp, the power amp is operating closer to its max output level hence the distortion from the power amp will also be higher. This will have its detrimental effect on SQ.
So, there is a sweet spot for the volume control - I;ve found it to be in the 12 o'clock to 2 o'clock position (if your source allows such a thing) - where the SNR is high enough & the noise (while higher) is not high enough to degrade the SQ. I also found that each recording (CD) has its optimum volume control where it sounds the best. This, of course, depends on system-system.