Midi is a protocol, meaning it is a language that equipment uses to communicate with the computer. They're generally cheap because they are essentially triggers and don't have very complicated analog circuitry.
Midi DJ packages aren't all that expensive and can be picked up with a small pay check; they're not going to break the bank.
There’s not much to say other than you get a lot for a relatively inexpensive price.
DJ packages are good when you're just a beginner and don't want to invest a TON into your equipment. The platters are usually small but don’t let this scare you. Midi controllers are being used more and more and they are becoming more robust and feature rich as time progresses. The one benefit is being able to play digital music and the lightweight consoles make it easy to go mobile
On the other hand, you could do better by mixing and matching your gear from items you find elsewhere online (think of it like building your own computer).
Because Djing can be a big investment, packages can be great for those that are on a budget and have been itching to get started. It's totally up to you.