This sentence is usually used at the end of letters or emails. This is a fixed format for writing letters or emails. Put it above the writer's signature to express the sender's blessing and condolences to the recipient, which probably means "I wish you all the best"
Generally speaking, this sentence is often used if the relationship between the two parties is close, or if the writer is a polite and kind person. Different kinds of letters and emails use different phrases. For example, business letters will end with "seriously" and some friends will end with "greetings". It should be noted that after these phrases, you should add a ","and then write your signature on a new line.
Don't ask why this is so, just the letter format, apply it to each other ~