OK, the buffer that you find in certain nutes help to stabilize that particular nute at a PH that it works well with, but sometimes the buffer in nutes can take the PH too low, for instance I use a nute with a built in PH buffer and it takes the PH all the way down to 4.5 sometimes and that being too low, I have to use my PH up and bring it back up to 5.7 or what ever I am wanting at that time. PH down helps when you have a PH that is to high, in hydro, 6.0/6.2 is around as high as you want to go IMO and staying under 6.0 is really best and above 5.3/5.5.
Those solution packages you got with your pen are for testing and adjusting your pen, after a while, sometimes a week, sometimes a month, it depends on the quality of your pen, you will want to test your pens calibration by poring that liquid into a little cup/bowl/lid of somesoart and placing the pen into the solution, the pen should read the same PH or TDS of the solution that you used, example, PH 4 solution you should get a reading of 4. The same goe's for TDS.
Hope this helps some
Adieu