SpecID can come in very useful if you want to maximize control of the amount of capital gain/loss that you take on when partially selling your position after dollar cost averaging or otherwise buying into it over many purchases.
For example, assume you've been dollar cost averaging into a security over the course of 5 years, and now want to start selling. You can use SpecID to sell only the specific lots that you bought between 1 and 2 years ago, so that you get taxed at long term gain rates, but otherwise realize the least amount of gains of all your lots (assuming the security has been appreciating at a stable rate). Neither FIFO nor LIFO would allow you to do that: FIFO would grab the oldest, most appreciated lots, while LIFO would grab the lots that have not yet aged into LT gain.
So that's an interesting point that I never actually realized (pun intended 1000%)... is that FIFO/LIFO doesn't actually change your profit amount when the position is fully closed, it only matters, as you said, when "showing" that profit, ie... if you are trading over multiple years and you need to do your taxes, the tax years up front might show more profit, but less in later years, or vice versa, but over all the years the profit would be the same. That's obvious in retrospect, and I can't believe I didn't get that before. Thanks!