There are three reasons that I can think of.
Firstly, a lenited f is written fh and is silent. We use the convention of always marking lenition with an h. This makes life easy. If you saw
an uil you would have no idea what uil meant and you would not even be able to find it in the dictionary. We write an fhuil so it is clear what it means and you can look it up in a dictionary if necessary. This contrasts with Welsh, where the same thing happens with g but they do not write the silent g. So if you see ei wisg you may not recognise the word wisg and it is not in any dictionary. The actually word is gwisg but the g went silent when it lenited. Give me Gaelic any time. (This actually means 'his uniform' so 'his' lenites in Welsh just like it does in Gaelic.)
Secondly, some letters have gone silent over time. Blàth would originally have been pronounced with the th as in English bath. Gaelic, Irish and English are very unusual in not updating their spelling system.
Thirdly, a lot of vowels are added because of 'the spelling rule', to ensure that every consonant is marked as broad or slender. Compare the Old Irish fer with Gaelic fear, where the a has been added to make sure you know the r is broad (even though it makes almost no difference with r).
If anyone can think of any other reasons, please post.
I wonder if it wouldnt be helpful for everyone to know, yes, there are regional differences, and this was done by volunteers. That information shifted how i feel during the lessons. More like, here we all are, doing the best we can. Which is perhaps an even more helpful gift than the Gaelic!
Blath is pronounced two ways by the voices - uwast and bla How is it to be pronounced? Are there regional differences or something - am finding this happening with words often - two or more versions of pronunciations. Is there anywhere that one could go to find an agreed upon pronunciation of words that are confusing? Thanks. I LOVE Gaelic!! This is a wonderful program!!