Having 1BTC out of the whole 21 million coins is the dream of every cryptocurrency investor, so why plan investing in different coins when you can just hold a signed coin that outshines every other cryptocurrency in the market?
Though it's not necessary if you are holding Bitcoins, he is probably looking to diversify his portfolio by adding different altcoins alongside Bitcoin. Most people do that because of the growth potential different cryptocurrencies carry which means that ETH might be able to have a higher growth percentage compared to Bitcoin during the bull run, which means if you have $100 invested in BTC and $100 in ETH, by the time you sell them both, if you get $400 from BTC, you might get $600 or more from your investment in ETH.
However, I think that one should try and gather some extra money and use that for altcoin investments instead of selling Bitcoins and using the money for buying altcoins since Bitcoin is also going to go up for the market to go up, and when that happens, you will already get good returns on your Bitcoin holdings.