Don't worry, it's not anything major, it's just a complimentary close to show your politeness and appreciation. You can also use "Best Regards,", "Kind Regards,", "Take Care", even "Cheers," or whatever

All emails in my work follow a template like below:

Hi XXX,

balabalabala......

Thanks,

Bill

I think it is same meaning. :)

"Sincerely yours" is more to common, and yes this used at the end of a letter.

And most importantly, is it must it be added to the end of a letter?

I have been confused about it for a long time. tks for ur help, good luck to u.

You can use any..

You may also use the following:

"Yours faithfully," --> British

"Sincerely," --> American

"Sincerely yours," --> American

"Yours trully," --> American

"Respectfully," --> Common

"Yours sincerely," --> Common