Sorry for the ignorance but I need a little help in understanding why a plywood board needs epoxy coating.

Is it for strength?

Or to make a smooth, beautiful surface?

What are the real benefits?

Can I get away with just 2 coats or so of varnish onto the naked ply and if so, what's the best type?

Any thoughts at all are hugely appreciated.

Course you can use varnish .You will need more than two coats to get a decent finish ,more like six plus . Epoxy is harder wearing .Used as a flow coat it can be done in one hit if done correctly . Good varnish ,two pack polyurathane . floor varnish or yacht varnish .Epifanes .

6 coats are a Minimum for poly, 1 spit coat (half thinner, half poly) then 5 full coats. UV resistance is not needed you dont leave your board out 24/7, so skipp to spar and marine poly, Use the fastest drying poly you can get Minwax makes a good fastdry poly, recoat in 2 hours. It takes 2 days to finish with fast dry poly or epoxy.
You could get by with 2 coats of poly, as long as you really wetted the wood out with the spit coat, but the board wont have a finish and just be waterproof.

Thanks crabman and bay surfer.

You're doing a (light) sand between each coat right?

I only sand after the SPIT coat, after that just lay it on every 4 hours. If using Epoxy and you want a Sacraficial or UV protection coat of poly. You MUST WASH THE EPOXY WITH SOAP AND WATER TWICE, and then sand lightly or the Poly will NEVER dry/cure.
If you want colored board, add a little oil paint or have the hardware store add pigment to the poly, they mite complain a little but those bright base pigments are really bright.

No ,I do one hit flow coat with clear polyester film .When cured I take off the polyester film and its finished ,no sanding nothing .Ido my graphics at the same time .Inever sand etc once its done .It comes up like glass . You cannot get the same finish even if you tried spraying .

No need for epoxy in a ply board. The wood will soak up too much resin and the board will become very heavy (I learned this the hard way ). A couple coats of Polyurethane Varnish or a Polyurathane boat paint will be much lighter and give you sufficient protection from water.