So when you buy your ETH using the ETH/BTC pair and BTC goes on a bull run, it makes your ETH (or any other coin paired with BTC) automatically go up in value as well assuming the price of ETH is the same. So in terms of strategy, it might make sense to trade your altcoins with the pair that is most bullish (or the least bearish) for that month of the major 3 pairs (BTC, ETH, USDT). However, the USDT pairs have the least liquidity. Hope that helps.