I have started on grade 6 theory and am exploring figured bass. I am enjoying learning about it and find it interesting to do but am struggling to find a practical application for this. I really would appreciate some help in how I can apply the theory from figured bass to practice.

If you ever play continuo (e.g. harpsichord, chamber organ, archlute, theorbo) in a baroque concert, it will be very useful.

I don't do any of these things Tenor Viol. So is it actually a waste of my time learning it?

Rather few amateur keyboard players can realise figured bass at sight, though any professional specialising in the baroque will do so. Most modern editions will give you a full two staves of keyboard part, with the figures realised. However, it may be too sparse or, more likely, too full for the circumstances of the performance, so it can be useful to know what the figures are (curses on any publisher that leaves them out), so as to amend the part in conformity with the composer's requirements. Even worse, some published realisations contradict the figures, so must be corrected.

Right now, I am producing a set of parts for a work by J S Bach* from an edition that gives figured bass but no realisation, and I don't know at this stage who will be playing it, so my version has to have a realisation, especially in the bars where only the soloist and the continuo (organ and double bass in this piece) are playing.

* The solo cantata "Ich habe genug", which I am arranging as a concertino for oboe and bassoon (the latter replacing the singer).

Not necessarily.
Understanding it on paper and making use in a practical sense are probably years apart for most people.

The only vaguely practical use I can see for most of us is in being able to work out the harmony if we come across a piece notated this way.....

Maybe for some this alternative way of notating harmony helps them to understand it better?

As you may have guessed, I'm interested in early music (c. pre 1750). If you play this music, often you will find you haven't got a published performing edition available or not for the instruments you want. In such cases, you may need to make your own performing edition, so you may be reading from a facsimile or early edition, so useful skill to have.

.... and it helps to understand harmony and the working of suspensions etc.

I agree, for most people it's a bit academic

My suggestion, in addition to the very practical reasons given already, is that working with music in the way it was notated by composers can only give you a better chance of really getting under the skin of the music. Thinking about all harmonies in relation to the bass, rather than prioritising chords, can give a different outlook on how the music works.