Why should I read manga?

The general order of depth that a series shows in its various versions is-

Light Novel(LN from now on)>Manga>Anime

Generally you would not notice the lack of depth between an Anime and a Manga, but u will notice it in comparison with the LN(of course assuming that the basic source material is a light novel).

Let me take up some cases-

Manga → Anime

For a series adapted directly from a Manga like Wotaku ni Koi wa Muzukashii Manga there generally isn’t much difference as the Animation studios have to take up the basic images, animate them and add voice.

Light Novel → Anime → (if) Manga

But for a LN based series they have to take up the important parts and consider what can be animated and what can’t be. Also a lot of times an LN based Anime is released and gets moderately successful, a Manga goes under serialization for the same series cleverly picking up the reviews for the anime and then deciding what parts to edit and continuing even further than the scope of the Anime.

Light Novel → Manga → Anime

This is just the reverse case of the previous one, but with one major difference that a lot of times you would see major differences between the Anime and Manga due to the fact that one is drawn and the other is animated. Animation is expensive and therefore you can’t take a big risk hence, resulting in some common cliché moments across anime. Also some parts have to be omitted due to the general constraints of 12 or 26 episodes. But Manga dues not require a lot of monetary efforts and therefore CAN stick with the storyline much closely(of course in the end it does depend on the mangaka, how closely they want to follow the original story).

Summing it up,

In the first case watching the anime is well enough (except in cases like Tokyo Ghoul, where they just shit everything up). While in the other 2 cases the Manga is of course better but it also matters upon how things were processed so you could do a little bit of research to find how much deviations the anime and manga offer from the original story.

ALSO, these are true for most of the series but you will find some exceptions.

One last thing, But as you can easily infer the most original content is always offered in the LN, but indeed its not possible to read the LN of every series.