A dance with its doubles - in this particular case (English Suite no. 1), Courante II and its doubles - is essentially a variation form. A double is a variation of the preceding dance. Also, often when Galanterien have been paired up, such as the Bourées here, the first is repeated da capo at the end of the second (as is the case with English Suite no. 1), much as Trios are used with Minuets, so we can say that at least some of the extra dances are used in effect to create larger forms.
I think, though, that the thing to remember here is that suites got their start (in the late 16th century) as essentially "playlists" of dances in the same key that were either thought to go together well, or were thematically related. The original core components were pavanes and galliardes, with other dances (bransles, etc.) thrown in.
Froberger standardised the core dances with then-current dances (allemande, courante, sarabande, gigue) in the mid-17th century, and his publishers standardised the order (he himself often used another order, such as gigue second and sarabande last), but I don't think the sense of a playlist of dances ever really went away. If it struck a composer's fancy to keep courantes or whatever going, why not?