This is easily answered with one word.  SECURITY.
We live in a troubled world where fraud runs rampant.  We require you to sign in or create an account to avoid fraudulent activity.  The benefits to this are you will be able to log back in at a later time to see your donation/offering history.  Our giving platform does not save your payment information on our systems.
This account also allows you to take advantage of features like an electronic check, recurring transactions, or give by text.  Give by text allows you to assign your text number to your account so all you have to do is send a text to the dollar amount the number the system provides you, and your giving is done for that amount without having to do anything else.