I never ever actually used an iPad before. My first one was iPad 1. That thing was gigantic and because of iOS 5, it could not do anything even in 2013. Skeuomorphism is nice though

and the system felt snappy. Then I got iPad mini 1. Again, could not do a whole lot with iOS 9, though I used to carry it around and play games on it as A5 was faster than iPhone 4’s A4.
In 2015 I got iPhone 6 Plus, and was blown away by its “large” screen. The iPad mini was forgotten because A8 was so much faster than A5. Sadly, that iPhone was stolen and I was forced to get iPhone 6s Plus, which then was used for 3 years until a recent water damage.
The biggest reason for me to not buy an iPad is I could not justify any use of having one. Sure, I might be able to use it as secondary display, but that’s pretty much it. No matlab, no industrial software support, cannot install arduino IDE, cannot use Microsoft Office efficiently, no Endnote support. The list goes on and on. iPhone 6s Plus is everything.
About a month ago, thanks for a seemingly nice deal (50% discount of an used iPad 2018 128GB wifi only), I started to think Maybe I could give iPad a try and see how it would fit my workflow and daily life. After some initial bumps and realisation that 2GB ram was insufficient, I end up going straight to iPad Pro 11” 2018, the iPad I am typing this post on. Apple Pencil 2 (!), A12X, large display, ProMotion, snappy iOS 13 and massively improved iPadOS all let me believe iPad has become much more useful than before. I even position iPad as video watching and comic reading device rather than on iPhone XS Max simply because of the massive display. It is also easy for my eyes as I am also near sighted. Yes, I hold that iPad and use it while lying on the bed. I just could not resist the large screen and overall better experience. I even jump and buy Apple keyboard folio, which is a nice keyboard in many ways (though I wish it has backlight).
To summarise that, I now find iPhone screen pathetically too small to do web browsing and video watching. Sure, there are gonna be massive letterboxes, but I can stand away from iPad to watch while not really an option for iPhone. I end up only using iPhone for playing certain games and doing things iPad simply cannot (control Apple Watch, phone call, text message and do NFC Apple Pay). I have cellular on iPad and Apple Watch as well, meaning they are independent to each other to an extent, and can act as a backup device for emergency communications.