Believe it or not, you're actually asking 2 different questions here. Secondary is one thing, using a barrel is another. Secondary with cider is really only needed to get the cider off the sediment (lees/trub).
Racking to secondary, regardless of the container, will get the cider off the sediment. These days, the barrel is used mainly for aging & flavour by homebrewers, but they still cask condition in Europe, mostly in the UK.
Different types of oak & different levels of toast (charring the inside of the barrel), as well as the length of time the cider is in the barrel, will add differing flavours to the cider.
Most homebrewers/winemakers forego the actual barrels as they're quite expensive and there are other, much less expensive alternatives to the barrel. Oak cubes, staves & spirals simply placed in the carbouy will add those same flavours to cider/wine/mead/beer; and cost only a few dollars, where a barrel could cost you a few hundred.
I like to use secondary with cider, as sometimes letting it sit on the sediment MIGHT produce some off flavours; but then I also like to age my cider for a year, whereas most (not all) cidermakers consume their product much earlier.
I'd say try some oak spirals or cubes & see if you like the results before investing in a barrel. If you like the results & can afford the barrel, then go for it!
Regards, GF.