I guess this suffix "-bar" literally represents "-able" in every English adjectives that end with "-able/-ble". Another example, like in this word "verfügbar" (which means "available"). I could be wrong, but it did help me a lot with remembering new German words, xD
Spot on. It doesn't alway fit perfectly, for example here we have sichtbar=sight-able=visible but you can usually work out the meaning from German to English and when speaking you can usually get away with "just sticking a -bar" on the end" even if you're not sure it'ts a real word.
does it accept "he's in view/sight" which I think would be the usual translation for "Er ist sichtbar"?
It does to me, too, but without context one cannot dismiss it as a wrong translation.
Oddly enough, they accept "Er ist sicht Bär" as a correct answer with a spelling error. Which it is, but since it spells out other words it shouldn't be accepted.
Why do they teach us these sort of words? Wouldn't it be better if we learned more common words like funny or worried?
"Sichtbar" is a fairly common word -- a lot more common in fact than translations of funny and worried:
"sichtbar" -- 0.0040%
"komisch" -- 0.00035%
"lustig" -- 0.00045%
"besorgt" -- 0.00070%
"sorgenvoll" -- 0.000022%
(Source: Google Ngram German corpus 2000-2009)
"Komisch" is an appropriate translation of funny in the sense of comedic, but also in the sense of strange. "Lustig" matches the meaning of funny as in a funny joke. "Besorgt" and "sorgenvoll" are both used for worried, but "besorgt" is also the past perfect of "besorgen" (to obtain).
Even if it was uncommon, I would say that there is nothing wrong with learning an uncommon word from time to time.