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TextSubstitution must be constructed with a Text type, which indicates a value that contains a unicode string in a manner that is compatible with both Python 2 and Python 3

Also, TextSubstitution inherits from Substitution, which provides a uniform interface for the runtime substitution of arguments of different types.

TextSubstitution must be constructed with a Text type, which indicates a value that contains a unicode unicode string in a manner that is compatible with both Python 2 and Python 3

Also, TextSubstitution inherits from Substitution, which provides a uniform interface for the runtime substitution of arguments of different types.

TextSubstitution is usually used to wrap literals in the launch description so they can be concatenated with other substitutions - but these substitutions must only be evaluated when the specific launch description is executed. In that context, it provides a uniform interface for all "substitutions".

https://github.com/ros2/launch/blob/b59bb6b88b40eaa82870542d93bdfb1a033d05c3/launch/doc/source/architecture.rst#substitutions

A partial purpose is that TextSubstitution must be constructed with a Text type, which indicates a value that contains a unicode string in a manner that is compatible with both Python 2 and Python 33 - so to work-around the differences in string handling/usage in Python2/3.