the thing is, with alts even though sometimes the ROI is indeed multiple folds of btc, but it also carries risk, you could judge if you happened to hold coins like luna and ftt i'm sure by now you're already broke but if you hold btc its just losing some fraction of its value.
there are several occassion that I do the same strategy buying bitcoin but also buying altcoins for other option.
and this is what have been called diversification .
make sure we need to do research before determining which altcoin we invest in,
occasionally it is important to carry out a diversification strategy.