CLL explains that in 3.3.:
“The apostrophe represents a phoneme similar to a short, breathy English “h”, (IPA [h]). The letter “h” is not used to represent this sound for two reasons: primarily in order to simplify explanations of the morphology, but also because the sound is very common, and the apostrophe is a visually lightweight representation of it. The apostrophe sound is a consonant in nature, but is not treated as either a consonant or a vowel for purposes of Lojban morphology (word-formation), which is explained in Chapter 4. In addition, the apostrophe visually parallels the comma and the period, which are also used (in different ways) to separate syllables.”
I used to write Lojban with ' and h at some point and it looked quite nice.
—mi'e la uakci