If you want to access videos with using Photos, Matti's suggestion is great. I have the video player VLC on my iPad to view videos in the Files app. Videos can also be played with the built-in Quick Look.
The advantage to having pictures and videos in Photos is that you have access to both edited versions and originals, you can use powerful search tools and organizationals devices, and without using extra storage, you can have multiple copies in different albums so you find what you want quickly. But some of these advantages don't apply so well for videos, so keeping then in Files can make sense.
Videos often use lots of storage space, and you should never pack a phone's storage more than 90% full. The Files app gives access to iCloud Drive, so videos can be kept off the iPad until they are downloaded for viewing. Of course, there needs to be room in the iPad's storage to hold the video that's being run.