Well obviously at the time OP posted it was a bad time to buy since its dumped since. I think now is good though because both btc and eth are overdue for a pump in the short term and their medium and long term prospects are also great.
You buy when price seems to have find a bottom on a certain level of a downwards move. Traders call this BTFD "buy the f***ing dip".Or you buy constantly a bit of BTC no matter what the price is, if you believe it will go up in the long run due to it's fundamentals. These are just two possibilities when/why you can/should buy. There are a few more and many people have different reasons to do so, depending on their investment/trading strategy.