Because it is NIST traceable, so the calibration is certified. Even inexpensive liquid in glass NIST traceable thermometers come with guarantees of their accuracy across the range, ie accurate to .5 degrees C, 1 degree C below 0 C and 1 above 70C. That is far better than one could hope for doing it at home, and even if one did succeed in calibrating to that accuracy at home they would still need a NIST traceable thermometer (or equivalent, such as ISO) to verify the calibration against.
Well, that is certainly an example (e.g., not i.e.) of NIST tracibility, but you're overstating what it is.
Having witnessed NIST traceable calibrations in-person, I can tell you it isn't as impressive as you make it sound. The values don't mean a whole lot outside of the calibration points. The assumption is that if you know enough about your equipment, you can fill in the gaps.
The claim for those thermometers you posted is that they are calibrated against NIST traceable equipment but without 1. A calibration certificate and 2. A calibration certificate that hasn't exceeded its 1 year life span, it itself doesn't come with guarantees and really isn't suitable for what you described.
If you've never seen a bulb thermometer calibration, it pretty much goes like this:
1. Put in cold liquid; scratch the glass.
2. Put in hot liquid; scratch the glass.
3. Measure the distance between the scratches and pick the closest silk screen for that length.
4. Line up the silk screen with the appropriate scratch and go to town.
With even the utmost care there is a lot of room for error in a bulb thermometer. It is no wonder they are notoriously inaccurate and it is why they aren't sold with calibration certs, at least, not at this price point.
You'd be worse off using one of these thermometers as a calibration reference than an air-pressure adjusted boil test.
If you had a thermometer with an unexpired certification itself within the temperature range you're interested in, then maybe you should go that route.
The only reason you would NEED to check your calibration against NIST tracible equipment is if you needed it as an item to check off as part of government required regulations. Without a cert, the thermometer you posted doesn't fit the bill regardless.
