Download question: why is Wifi needed for download with Ipad and Iphone?

If you download using wifi, you are using up your data allowance with your broadband ISP. Using cellular data your are using your data allocation from your cellular data plan.

Both almost certainly have data limits, but your landline connection is almost certainly cheaper than the cellular connection, and probably less limited in terms of how much you are allowed to download.

And of course your cellular data plan allows you to make use of the internet when you are out of range of a wifi alternative.

Anything you choose to download from iTunes tells you the file size on the page describing it.

As you have discovered, some larger files need you to connect to WiFi to download them. With the cell plan, you can download most files, watch videos, etc without connecting to WiFi. The only person who can tell you if you need the cellular plan is you, how often do you use it, does it meet your needs, etc. When you try to download a file via celluar and it is too big, you will get a notification and the file will remain in a pending status for you to download later. Just swing by a WiFi hotspot to complete the download, home network, Apple Store, coffee shops, etc. Some apps have been written for the iPad only and those cannot be downloaded on the iPhone and are marked as such, that has nothing to do with your WiFi/cellular question.