Mostly with DVDs so I can load them onto USB drives and play them with a WDTV media player, or smart TV. Why? The WDTV could shrink the picture to compensate for overscan on my old CRT TV, and zoom letterboxed movies to properly fill the screen, and, by remuxing through Mkvtoolnix (free), fix aspect ratio errors. And, for TV series, a folder full of mkvs is more convenient than keeping track of which episode on which disc to watch next. And then, sometimes I rent or borrow discs but don't have time to watch them right away.
I mainly use Makemkv (free), which decrypts DVDs (when necessary) and (unsurprisingly) converts the video to mkv. If I want to create an image or folder of the whole disc (best for bonus discs with lots of extras), AnyDVD (not free) or DVDFab Passkey (free).
I don't recommend DVDDecrypter since it hasn't been updated for a very long time, and DVD publishers have come up with new types of copy protection since then. But it can still be useful for those rare DVDs that have been authored in some peculiar way that confuses Makemkv.
A final good reason to rip discs is for HD DVDs: the most common (Toshiba, Venturer) standalone players didn't have 1080p outputs. Sometimes I had to rip to m2ts rather than mkv to get smooth playback with the WD TV Live, and Dolby Digital Plus soundtracks would need to be converted with eac3to.

No but I have a huge amount of mp4 files.

I know laserdisc used to have rot problems but never for dvds. I labeled some blanks with ink jet labels back in the early 2000s and it would cause wobble problems. To get the sticky labels off completely I'd boil the discs I kid you not. And they're still playing fine to this day.
Tell you what…When I send my ex-wife her money…you can lick the stamps.

Some of my TV series. Some of my DVD sets are already getting disc rot sot I rip them to preserve the shows.
Grade "A" Fully Loaded
"Sexy as Hell"

Yes. I have 2 USB sticks connected to my Samsung 60 inch. Lots of High Def 1080p movies and some DVDs I converted to mp4 using:
Smart Ripper free
Handbrake free
Tell you what…When I send my ex-wife her money…you can lick the stamps.

DVDDecrypter to rip the DVD
Vidcoder to convert to MP4
MakeMKV if you prefer that format
Are you okay?

VLC media player can convert files to different types.
Primates evolve over millions of years. I evolve in seconds.

This website is a very good resource for anything video format related:
https://www.videohelp.com/software
In the format listing, just click whatever you are looking for and it will show you software that will do the job, rated from best to worst.