I'll echo Wayne there. It's only in the last couple of years that I've started using Future (Klear is the name used in UK and some other countries, for the same product). I started using it as I could no longer obtain the usual clear varnish I used to use, and it is basically a clear acrylic varnish anyway. To obtain a true gloss finish, especially with an airbrush, requires more than one coat, but it is thin enough to spray from the bottle.
The main advantages are it's low price per ml compared to 'modelling' products - a large bottle is around £2 (approx $3.50 US) compared to the same, or more, for a very small bottle of 'modelling' varnish - and it's ability to be applied by paint brush and dry without brush marks.
Another use, much favoured by many modellers, is for giving a sparkling, clear appearance to transparent parts. I admit, I've only used it this way on a couple of occassions, and it has been fine. It can also be used for repairs to clear parts which have been scratched, or affected by adhesive; once the damage has been polished, an application of Future makes it like new.
One of the big disadvantges when used in modelling is it's ability to attract, and trap, tiny dust particles, which only become evident when the Future has dried.
I must admit that, although it is a suoer product, and dosen't mask or 'clog' fine detail, even after a number of coats, I am considering using a different product as a gloss coat, mainly because of the dust issue, and the sometimes patchy effect caused by temperature variations during drying. As a lot of my modelling products are now obtained via mail order, something that, in general, is relatively recent for me, I might as well order a varnish when ordering other products.
However, as a 'quick fix' for touch-ups etc, Future/Klear will still be in my paints rack. Also, I find it excellent for making various matt, semi matt and semi gloss clear finishes, just by adding a flatting agent as required. This dries, and sprays clear, without yellowing (to date anyway!), which is a big advantage over some commercially available matt finsihes, particularly in the ability to vary the surface sheen, even down to full matt.
Finally, for those really tricky joints with clear parts, I find that Future can be used in some instances, and certainly as a 'second string', where a thin run of the liquid around a joint will definitely seal and hold the already fixed part.
I recently knocked over a new bottle of Klear and, having cleaned up the mess, thought I'd removed all traces. That is, until I went to move a 'desk tidy' a couple of days later, and found it was stuck fast to the desk!