|A frontispiece from a Jawi edition of Sejarah Melayu.|
This makes it easy to get by without vowels, or at least without making the vowels explicit, and it means that there can be significant variation in the value given to the vowels in different dialects without affecting the nature of the written language. This is one reason why scripts for Afroasiatic languages (some of the earliest in the world, including Egyptian, source of most of the extant scripts of our planet, including this one) tend to consist of consonants, with vowels added as superfluous afterthoughts. It's also why the alphabet, marking vowels and consonants equally, is a relatively recent invention, and one that took place largely outside of Afroasiatic-speaking communities.
Well, vowels are pretty important in Malay and other Austronesian languages. Afroasiatic languages are centred on consonantal roots, but Austronesian languages tend to employ CVCV or CVC forms, with vowels carrying much of the meaning-burden. Malay, which is actually fairly tolerant of consonant clusters, still gives a lot of weight to vowels and their values in differentiating between words. The words tumbuh (to grow) and tambah (to add) differ only in their vowels, and since it looks really awkward to mark every vowel using the Arabic script, standardised absences had to be invented. In Jawi tambah is written tm-b-h, while tumbuh is written tm-b-u-h (where tm is a peculiar form indicating t followed by m).
Knowing the letters is easy, but that's only half the battle: you also have to know when to indicate the presence of a vowel and when to leave it implicit, and that makes it a little trickier than it ought to be. On top of that, Jawi only gives you three vowels to work with: one representing /a/; another representing /i/, /e/, /ai/, and /ei/; and a third covering /o/ and /u/. Needless to say, Malay has more than three vowels, and the difference between /o/ and /u/ could serve to differentiate the meanings of two or more words (Jawi is really testing my intuitive grasp of Malay/Indonesian, which is great). I'm at the point where I know all the letters, how to put them together, and what words they probably represent, but I can't reliably sight-read Jawi. I find myself pausing over words I certainly know.
I can understand why the Roman script has almost entirely (Brunei is an exception) replaced Jawi in the Malay world - it's an obvious improvement for a language like that. But it's still worthwhile to learn Jawi: it opens up a whole new world of post-1400 CE sources for the study of the Malay-speaking world, including Ternate and Tidore and the rest of the Muslim east (the earliest inscription in Jawi is the Terengganu Stone, dated 1303 CE, but common use of the script begins some time after this). I should also say that I find it fun to learn new scripts, and the Arabic abjad is in such common use in so many places that it will probably pay off to learn it properly.