I am by no means a heads-up specialist, but when stacks get shorter than 20BB, limping into the pot protect you from getting jammed on and losing 2BB or more every time, you need to fold. And I think, this is the main reason, why people do it. Its probably also, what solvers suggest as the optimal strategy.
You have paid 0,5BB already, so limping gives you a very good price, especially if there is a big ante. Many live tournaments have moved to a 1BB BNT ante, and that really matter, when it gets down to heads up. If you have to call 0,5BB to win 3,0BB in position, you cant fold any hand at all, and by limping with strong hands as well, you protect the weaker part of your range.
I am sure, some tournament specialists can add more to the subject, but I think, this is the basic theory behind a limping strategy heads up. A lot of this will also apply to blind vs. blind situations in 3+ handed situations apart from the fact, that you are going to be out of position postflop.