What large storage model Androids mostly do (all ought to) is overlay storage so that /data and /sdcard draw from the same pool. Iow, on my 32 GB phone, 25 is free for me - I can load that much up for apps, or use few apps and load it up with pictures and music.
And while the two draw from the same pool, they can't be visible to one another or you'd have a security nightmare.
Linux has advanced filesystem types that makes that possible. And less advanced, but still more advanced filesystem types than Windows, for when /data and an internal /sdcard are side by side. Android is a Linux distribution so it uses those methods.
None of which is fully supported in Windows or on a Mac.
Enter MTP - as Wikipedia explains -
"The Media Transfer Protocol (MTP) is an extension to the Picture Transfer Protocol (PTP) communications protocol that allows media files to be transferred atomically to and from portable devices.[1] Whereas PTP was designed for downloading photographs from digital cameras, Media Transfer Protocol allows the transfer of music files on digital audio players and media files on portable media players, as well as personal information on personal digital assistants. MTP is a key part of WMDRM10-PD,[1] a digital rights management (DRM) service for the Windows Media platform."
So instead of mounting the storage like a USB stick with UMS, files come and go through a software service that lets your phone and PC access the same storage at the same time.
And unlike UMS, you don't have to unmount/eject an MTP device before unplugging from your PC.
Which leads us to the cases of those users that argue that's not really required - and then blame their phones when the background media scanner goes nuts eating the battery if the sd card gets a corrupted file.
Or the people who read that formatting their sd card is a good idea, do it while the phone is plugged in to the PC and get upset because their pictures disappeared.
Because MTP doesn't care about filesystem types and because makers hate support and unhappy customers - presto - the actual sd card gets added to the MTP mix and buh bye UMS. And the makers have one less software component to maintain when coming up with new phones.
That's all of the advantages.
The disadvantages to non-Microsoft users range from nuisance to headache. It's only fairly recently that Linux bundled support for it by default and Macs still don't.
The disadvantage when all you need is fast access to your SD card is that you can't have it, you have to go through all manner of software on your phone and pc just to move files.
The good news is another pain in the neck solution but it works - get an SD card reader for your pc and physically move the card from phone to pc and back for high speed file transfers. EDIT - I'd advise to use sparingly because tiny parts are fragile.
