For all Gmail has changed over the years, there are still bits of it that are a bit old. For example, if you send a video attachment, you still have to download it onto your computer first in order to watch it.
That said it’s not going to change terribly much. Videos over 25 MB are uploaded onto Google Drive automatically, which means they would have streamed anyway. But for smaller clips, like some funny recording you want to send your family – I can’t tell you the number of times I’ve had to explain how to open a downloaded attachment – it’ll at least make your life a bit easier. Google says the change is rolling out to Gmail over the next 15 days or so.