What's the significance of the banner and it's bearer? [SM2/Helsreach spoilers]

In many 40K stories, the chapters and regiments banner comes into play and it's significance is emphasised by how important the bearers role is and how it's like PRIORITY #1 that the banner doesn't fall or get lost.

I'm reading The First Wall, and it's talked about how much of an honour and a responsibility it is for an army trooper to look after and bear the banner.

In Helsreach, Grimaldus' best buddy (his name escapes me, forgive me) bears the banner, and fights one handed so he doesn't let the banner fall. I'm assuming if he lost his good arm, he would resort to headbutting rather than drop the banner.

In SM2, the planet is coming apart and a Lord of Change is causing carnage on the horizon... but HOLD UP! The banner is in danger! Get to the banner! Defend the banner!

My knowledge of military history (irl) is lacking but I assume this is a call back to like, knightly orders and medieval armies?... or do modern armies today have banners which are super important to them? I watched Generation Kill (Iraq war) where a scene has US Marines misplace their "company colors" and it's kind of a big deal for the company.

So I guess the answer to my query might be based in real life, but it is 40K which has drawn my attention to it...