If you run the below code, Exist method will be passed and the value will be set into webedit.
Suppose, if you run the code for negative condition (I.e. You are going to look for webedit in the first column where it does not exist), you will get an error as same as below.
If obj.ChildItem(1,2,"WebEdit",0).Exist(0) Then
obj.ChildItem(1,2,"WebEdit",0).Set "QTP"
End If

To avoid this problem, use ChildItemCount method instead of ChildItem for the above criteria.
Now, you do not get the error for negative case.
If obj.ChildItemCount(1,1,"WebEdit")>0 Then
obj.ChildItem(1,1,"WebEdit",0).Set "QTP"
End If