Hope or a hope that is seen

Hi. In Romans 8:24 of the Bible in the New International Version, it is written:

For in this hope we were saved. But hope that is seen is no hope at all. Who hopes for what they already have?

If the part "But hope that is seen is no hope at all" were written "But a hope that is seen is no hope at all," what is the difference? Thank you in advance for your help.