You can use an iPhone with the cell part off, but I do not think you can activate an iPhone without a sim card. You may use an existing sim card from another phone, of course, but it needs to be a nano sim and I think it will mess up older services if you use it to activate a new account. Basically Apple wants the iPhone to be used as a phone with a valid cell number uniquely linked to a single individual. You can probably find a way around that, but it is not going to be convenient.
Basically, these devices: smartphones and tablets, from Apple and Android, are designed to be used in very particular ways. They are designed to be linked to a single human, identified by a single phone number linked to their official identity (e.g. social security number). Then, that person is expected to mainly use them to consume subscriptions (communications, music, videos, news, data storage) and buy software from the approved seller. They also identify you for advertising on the internet (more so under Android, obviously, but Apple also has iads). I seem to recall that the manufacturers expect around 100$ of revenue a year per device.
They can be used professionally, but this usually means that your company has to pay to have the system set up for their needs and only large companies can really do that.
The tablets, especially the iPads, are aslo recently marketed as a device for creators. I am not so sure how much of that is true. Not that you cannot draw or make music with them (you can and they are very good), but history shows that the phones have also been marketed for creation: photography and video. The setup is however designed that the easiest is to share your creations for free on several social networks.
So, basically, the creators of the mobile operating systems have carefuly designed their system to give you little privacy, have you pay subscriptions to consume other people’s creations and share yours for free. And that is what the vast majority of the over 2 billions users of smartphone do. By design.