When Philips introduced the Digital Video Cartridge in 1993 they released a new category of titles that were compatible now with the CD-i format: Movies. Before the Digital Video Cartridge this was simply not possible. The Video CD standard however was not yet finalized in the White Book. The movies that Philips released on CD-i were at first coded in the 'Digital Video on CD-i' format. When the White Book standard was all standardized, Philips switched to the Video CD standard as well to have Video CD compatible with a broader range of players. This means CD-i movies after April 1994 were released as Video CD; CD-i movies released before this date were printed in the 'Digital Video on CD-i' format. Those early movies, like 'Top Gun', are not compatible with Video CD players, you'll need a specific CD-i player to play this on.
The introduction of the Digital Video Cartridge did not only make it possible to play movies on CD-i: The expansion unit contained a 32 bit RISC processor and 1 MB of RAM to provide MPEG-1 decoding.
This additional memory was also available for other CD-i applications like games: With the extra RAM better scrolling was possible, additional features like extra SFX sounds or background music. Games could benefit from the extra memory so more things could happen on screen at the same time as more could be preloaded; the extra caching made it possible that loading times were shorter.
The Museum of obsolete Media about the formats and its differences: "One of the two differences between the formats Video CD and 'Digital Video on CD-i' was the resolution, that was slightly higher on 'Digital Video on CD-i' (384×288 instead of 352×288 for Video CD). When a Video CD is played on a CD-i player, slightly larger pixels are displayed to fill the screen.
Only around 20 movies (around 10% of the whole Philips VCD catalogue) were released on 'Digital Video on CD-i' before Philips switched to Video CD for distributing movies in 1994."
[Thanks, Museum of obsolete Media]