Vexillology - the study of flags.

Traditionally, flags were used as a symbol of identification of countries, nations, organizations, political parties, etc.

From the national perspective, flags were often used in the battlefield to organize troops and act as a banner for specific landmarks (such as a castle or a palace). Flags were a way of representing a people, and thus uniting them.

As time progressed, flags became integrated into society in various ways. The best example of this that I can think of is that of naval maritime communication flags.

Do flags serve a purpose in today's modern world? Are they still valuable, and if so, for what? Or are they merely an artifact of the past, something that doesn't necessarily need to be disposed of but has no practical purpose?

I believe flags still endure as a political/national identification symbol (for unification). Maritime flags are used out of tradition and procedure, although I suppose in a cinch they would work as an emergency communication device. You still see the flags put on top of tall buildings so that planes don't fly into them.

But as a battlefield device, are flags really necessary? They seem to be only an aesthetic of the military - place the flag on conquered territory; although I suppose these flags would be used as a signaling device to friendlies and the surrounding people to notify them who is in charge (hence why undercover units do not use flags).