that is only your choice based on your strategy, that doesn't mean it is the right move for everyone!

the main strategy that majority of traders use (which you can verify simply by looking at volumes and comparing alt/USD versus alt/BTC volumes) is to always go back to bitcoin because bitcoin for everyone is the long term investment which they know is going to go up in long term. so what they do is that for example they invest $1000 in bitcoin (get ~0.2BTC) and then use that to trade and in the end the goal is to turn that 0.2 into 0.5 then 1 then 2,... so that in the future when bitcoin reaches mass adoption and is $1 million they already have a big stash of bitcoins by only investing $1000.

in other words it only works if you expect the market to go down. if price rises (which is the usual case with BITCOIN price only) you will miss out and have to buy a much smaller amount with the same USD amount. like what we had this month where people were waiting around and price jumped up and now is 75% above the bottom where they sold!

that is mainly because majority of people don't trade in USD, they trade in BTC which is why whenever bitcoin price is rising they start dumping their altcoins for bitcoin to go back to bitcoin market with BTC in hand. but since this is not a 100% dump the price against BTC decreases and their price against USD increases slightly.

WARNING
none of these are US dollar so all these names are WRONG. they are all Tether or USDT which is a very risky altcoin and when you go to Tether instead of fiat (USD) then you are taking an additional HUGE RISK.