>>>It's a good thing to have?
As an analogy, consider the position if you were asked to act as guarantor for a friend who was renting a flat (so that, if they failed to pay the rent, you could then be forced to do so).
It would be a 'good thing' for the landlord (because they'd know that they'd always be able to get their rent). It would be a 'good thing' for your friend (because they'd be allowed to take on the tenancy). Whether or not it would be a 'good thing' for you is, however, questionable. You'd have the satisfaction of knowing that you'd helped a friend but also the worry about possibly be forced to pay their debts to the landlord if your friend defaulted.
Similarly, a letter of indemnity (which is much the same as the 'guarantee' in my example) is a 'good thing' for the person who is assured of getting their money if things go wrong, it's a 'good thing' for the person who the letter is providing backing for (because they'll be able to enter into whatever type of contract it is that's involved) but it could potentially be a very 'bad thing' for the person writing the letter if they're later forced into settling someone else's debt!