This is a very simple 'wrapper' function. It takes a Boolean value (true of false) and returns that value. It can be useful if you want to use operators (e.g. 'and' or 'or') to combine various boolean expressions together, and you don't already have a function to provide the context.
Note: boolean operators can be combined in forming a (boolean) parameter to any function (that take a boolean parameter). There is nothing special about this function as far as that goes. So, for example,
=TextIf(Exists(%INDI.BIRT%) or Exists(%INDI.DEAT%),"Has Birth or Death", "Has neither Birth nor Death")
is fine. The first boolean parameter consists of the expression "Exists(%INDI.BIRT%) or Exists(%INDI.DEAT%)". There is no need to write the whole thing as
=TextIf(IsTrue(Exists(%INDI.BIRT%) or Exists(%INDI.DEAT%)),"Has Birth or Death", "Has neither Birth nor Death")
although of course you could. The real benefit of the IsTrue function is simply that it provides a convenient 'wrapper' function to hold together boolean expression, combined by operators, if this be needed.
=IsTrue(Exists(%INDI.BIRT%) or Exists(%INDI.DEAT%))