This is called Alif Khanjariyyah or Dagger Alif. It's not necessary at all as we don't really use it in daily writing or anything and many people complained about it but Duolingo keeps using it.
It is a guide that there is a long (a) with this consonant but it's hidden or not written. So, sort of a guide. Anyway, the problem is... it can't be found on regular keyboards at all - one has to use special means to type it down. It is completely impractical and unnecessary even yet Duolingo still keeps it.