Sales persons are necessary when continuing growth requires efforts that do not scale as effectively. Marketing and advertising campaigns work sufficiently well for b2c with 1-2 people but you hit a wall if you want to selling to, for example, the Pentagon because the way those with the deepest pockets buy is nothing like how most efficient individuals and organizations buy. Groupon's massive needs for salespeople is an example of how even lower margin b2b even needs sales due to how scaling out b2b is so much less cost-effective. But for investors, landing a huge contract is not as much about the revenue as much as an indicator of stability in terms of both revenue and market signal (no Fortune 100 will buy some random fly-by-night vendor's stuff without code escrow even if it's the best thing ever because their sourcing overhead is so poor and big companies are immensely risk-averse by design).

Yea, looking at the employee page for Stack Overflow shows a vast majority of the staff being in sales.