Learn Turkish: Her devlet için bir bayrak gereklidir. - A flag is necessary for every state.

The suffix -dir is the factual or generalizing copula (meaning "is"). In this sentence, it is used because stating that every state needs a flag is a universal fact or an encyclopedic truth. While gerekli alone means "is necessary" in everyday speech, adding -dir makes it sound like an objective, formal rule.

Why does the sentence start with Her devlet için instead of the subject bir bayrak?

In Turkish, it is very common to set up the context or background information at the beginning of the sentence. Starting with Her devlet için (For every state) establishes the context first, before introducing the subject (bir bayrak) and the predicate (gereklidir) at the end.

I usually see gerek or lazım used for "need". What is the difference between gerek and gerekli?

Both can mean "necessary", but they function differently. Gerek is usually used in structures where someone needs to do an action (like gitmem gerek - I need to go). Gerekli is a standard adjective formed by adding the -li suffix to gerek. It is used here to describe the flag as a "necessary thing".

Does the postposition için (for) require a special case ending on the word devlet?