A person could say "..this...", show us an object, and continue the sentence or start a new one with "It is." Some examples that might work with this are:
"Now I want to show you this,it is an elephant skeleton."
"Look at this. It is an exotic flower."
"Take this. It is used to wash your hair." (I thought this might work because you said they had a little pause).
Also, the person speaking could have lost their trail of thought or forgotten what they are trying to say.
For example, you could be trying to explain your science project to a class. You start saying "This...", but change your mind and say "It is....". You may do this because you forgot what you were going to say after this or because you found a better sentence to begin with "it is."
The last thing that I think another user touched on as well is the form of speech "That it is.", which is a bit different. When you say 'that it is' you are saying "you're right, it is." For example, I could say "That is such a cute dog!" and you'd respond "That it is."