The "standard" helpers are mostly just a convenience; there's almost nothing there that can't be done with the corresponding functions in System, SysUtils, etc.
IMHO, their use depends more on preference than on anything else; sometimes you might find more useful or readable to do
SomeInt := SomeString.ToInteger;
and others it might seem more intuitive to write
SomeInt := StrToInt(SomeString);
Basically, they simply "convert" normal types in "almost-classes" (at least from a sintactical p.o.v)