Why does typing the return value of my function make the return value typed unknown?

why would i need to type the return value although i told typescript what will the function return?

You've told TypeScript that fetchCashTeams returns Promise.

You then said return response;

Therefore TypeScript checks that response is Promise and it isn't. It is Promise.

This is a check to prevent you from returning an invalid value.

response is Promise because httpRequest returns Promise and you didn't give it any hint as to what T is.

The simple way around this is to be explicit about that: httpRequest("/api/cash-teams", "GET");

This does mean that you are writing an assumption that you will get data in that format back, so you could make it more robust by using a guard function:

function isTeamTypeArray(value: any): value is TeamType[] {
   return Array.isArray(value) && 
       value.every(
           (team) => ('someField' in team && 'someOtherField' in team)
       );
}
export async function fetchCashTeams(): Promise {  
    const response = await httpRequest("/api/cash-teams", "GET");
    if (isTeamTypeArray(response)) {
        return response;
    }
    throw new Error("Value returned from web service was not a TeamType[]");
}

That is just a simple example of a Type Guard, you could make a more robust one by validating using a JSON Schema library.