Storing images and video on-chain is expensive. To support media in a decentralized way it must be stored elsewhere. IPFS would be added to the current options (Imgur, Youtube, etc).
The difference being that IPFS can be supported natively - i.e. you could upload files through Memo website / app.
Yes, that was the original plan. If you look at the source of the protocol page there's an action commented out for setting a base URL, basically a content hosting provider.
what's the difference between running a central server and running an IPFS node? You need a server anyhow. With IPFS people can help hosting their own files.
One benefit is that it doesn't require DNS or even a public server. You can run a local Memo client that uses IPFS. Another nice thing is it has built in easily parsable hashes.
Uploading directly integrates the media effort and the posting effort => will prompt more participation (less hesitation about going offsite to get URLs.