A flashtube t-scope comes with its own drivers to run the falshtubes.  Indeed, it probably also isequipped with its own timing circuits.  However, many t-scopes are very old (albeit still serviceable) and the timers are not especially accurate.  It makes a lot of sense to use a computer to do the timing if you know how to do real-time programming because you gain a lot of control over how the sequencing is done on the t-scope and, importantly, because you can also use the computer to collect reaction time data.

In order to collect reaction time data, you must start the computer timer with the onset or offset of the stimulus presented by the t-scope.  Below is an optical isolator that was used to interface a C-64 computer with an older model of t-scope.  You would want to use an optical isolator on t-scopes that use electronics that pre-dates TTL logic.