There are many theories on this. Hist sap is a pretty interesting thing in itself (in the upcoming ESO dlc [spoiler]a scholar compares amberplasm to creatia plasm from Oblivion, leaking in from another world to Tamriel through the bark of the Hist).[/spoiler]

In any case there's evidence of Argonians being born without access to Hist trees that appear normal but have difficult socially interacting with other Argonians when returning to Black Marsh (while interacting with other races just fine), which is indicative of Argonians being born as they appear, and not remaining physically undeveloped without licking the sap. This makes sense to me, if there are to be communities of Argonians that exist outside of Black Marsh (unless they are unable to replace themselves).

The Hist probably give them souls. In Lord of Souls/Infernal City the Argonian protagonist muses about vague dreams he's had in the past of things he didn't experience personally (along with swimming, and hatchlings-tadpoles imagery), and takes this as proof the Hist gives the Argonians souls, recycled from previous lives.

Licking the sap would connect them to the Hist, and is a trial for Argonians entering into adulthood in Black Marsh. I do not believe however that licking Hist sap is necessary for reproduction (though it should be noted that considering how widely varied Argonians are I also believe they exhibit traits of both warm blooded and cold blooded species, and can both give birth to live young as well as hatch out of eggs). Perhaps this can be extended into saying that some species of Argonians require Hist sap as part of their life cycles whereas others do not (and these others could create communities outside of Black Marsh.

Basically Argonians are a genetic experiment and can exhibit a wide variety of traits. Hist sap is for a connection to the Hist. Argonians may experience physical changes by ingesting sap, but there is evidence of physically normal Argonians who grew up out of Black Marsh (Huleeya in Morrowind, Ja-Reet in ESO); such Argonians likely will have trouble communicating with their kin in Black Marsh but otherwise have no particular problems communicating with other races. Hist sap may be necessary to sustain a viable Argonian community as well, and if that is true then Argonian communities living outside of Black Marsh are unlikely to thrive.

Some other commentary on this:

https://www.reddit.com/r/teslore/comments/4jk0iv/a_question_on_argonians_and_the_connection_with/