Comment by suprasanna

As a quick 'part answer' to your question, a lot of the reasoning behind so many rounds for many companies is actively choosing to stay unprofitable by pouring would-be profits (and new investment dollars) into gaining more customers. The idea being that as long as you "know" you can get your customer to be worth more than you paid to acquire her, eventually your company will be profitable once you slow down (or hit a 'max') with user acquisition.