When to make a small bet in two-player Haggis?
This question was discussed in this BGG thread.

The only answer I found at all convincing was: in a 3-player game, if your hand is marginal, the big bet may convince your opponents to put their own interests aside and work together to set you. If you make a small bet on a similar hand, your opponents might play less aggressively to defeat you--each trying to snag more point cards himself, say--leaving you with a slightly better chance of making the bet. Clearly, this does not apply to 2-player.

It was also pointed out that the 15-point bet allows a risk-averse novice to experiment with the bet mechanic without being buried for failure. This isn't strong general strategy but it's a reason you might see the small bet.