Why does this need to be a separate entry? Isn't it just {tlhap} + {-qa'}?

In the English–Klingon side of The Klingon Dictionary, there are numerous entries for English words that one might look up but which are word + suffix in Klingon. For instance, one might want to look up teach, but if the E-K dictionary only included words that led to Klingon stems, you wouldn't find teach. So it includes teach and says that it's ghojmoH. Likewise, you might want to look up ocean, but an ocean in Klingon is a bIQ'a', so the E-K dictionary lists it that way.

For symmetry, and because the extra information is sometimes useful, the Klingon–English dictionary includes all of the entries found in the E-K dictionary, even if they're not word stems in Klingon. In the K-E dictionary you'll find separate entries for ghojmoH and bIQ'a', even though these are really just word + suffix. A native Klingon (non-translation) dictionary might not list these words separately, but a translation dictionary does.

Including an entry for tlhapqa' is just continuing this trend. One might try to look up the word retrieve, but since it's not a bare stem in Klingon but a word + suffix, it appears as word + suffix. So the K-E dictionary is also updated to include tlhapqa' retrieve (etc.) because that's how it was done in TKD.

Even if you're using a lookup app instead of dictionary lists, if you search for retrieve, you won't find it unless tlhapqa' is a separate entry.

-- 
SuStel
http://trimboli.name