Whenever a daytime event, formality wise, does not require a Morning Coat; a state funeral or a royal christening does - the Stroller will be correct attire, echoing the seriousness of the occasion.
Interestingly, as far back as the stroller dates, it's a wonderful way to look non-standard and still not out of place. If you are attending a day wedding, and the dress code permits, a stroller is suit-like enough that you won't look like a fashion poof, and different enough to stand out just a little. @Kent Wang's stroller would be a good example...
As the subject of "strollers" has been brought up, here's a picture from August 1986, the last time I wore my grey and black striped trousers. I am also wearing an (almost) black Brooks Brothers jacket (part of a three piece Brooks Brothers "Special Order" tropical worsted suit that I got in 1976), a Brooks Brothers black, grey, and white striped tie, grey vest, and white OCBD shirt (yes, I'm wearing a button down shirt) and black Johnston & Murphy cap toe shoes. Since it's August, I am not wearing the grey spats that I got at a shop on Chicago in 1967.
Still have everything in this picture (except the shoes which have been replaced with a similar Allen-Edmonds pair) and everything still fits, but I don't think that I will have an occasion to wear the striped trousers (or the spats) again. Still wear the (almost) black suit to funerals (the only time that I wear it) and sometimes wear the grey vest with other things.
White vests (with dark suits) are for breakfast, grey vests (with dark jackets and striped trousers) are for lunch, and dinner jackets are for dinner time. Unlike some made up "rules" these are actual rules that were in a Rule Book.
Wearing a white vest in April 1968.........
(Also wearing the same Hamilton Railway Special pocket watch and chain and Santa Fe Railway employe lapel pin in all three pictures.)
White, grey and other waistcoats are fine with a dark suit almost any time - the misunderstanding is due to the fact that a Stroller (Black Jacket as it is called in the UK or City Dress or Stresemann in other parts of Europe) is a specific mode of dress which is worn correctly with a black SB or DB waistcoat, Of course these days one can do whatever one fancy.
My interpretation is different. The historical provenance for wearing waistcoats of gray or buff is strong. Stresemann himself wore it with a waistcoat of the same color as the jacket, true, but he also wore a wing-collar shirt. This hardly means that this also must be worn with it. Things are always evolving and men correctly assumed that the Stresemann, or troller, was a corollary of the morning suit. Morning suits were often paired with gray or buff waistcoats and the stroller followed suit.
See here the morning suit worn with light colored waistcoat and the stroller in the same. Both in historical context.