I have been working with Brides now for over twenty years advising on the dress & the styling to create the look each customer has in mind.
When we first started building up the showcase of bridal samples, making separates was not only logical, it was cost effective. Every time I added another dress combination of corset, skirt & sleeves/strap, I added more pieces that could be mixed & matched with pieces already in the collection. As we were exclusively making one of a kind gowns, having a collection of pieces offered up more looks for customers to try on & it worked really well.

Our Valentina corset with her original skirt (worn over & under the skirt|). With her strap & sleeve options demonstrating how Sleeves & Straps change the look of a dress.
Sleeves & straps:
From experience, sleeves are one of three things that change the look & mood of a dress more than anything else & it is perhaps why so few off the peg dresses come with sleeves. Manufacturers possibly do not want to limit a dress’s appeal. Yet a lot of brides want sleeves. In their most basic form, a straight sleeve will help disguise bingo wings (which are the obsession of many brides young & old & of every body shape), even a semi-transparent one. The simple addition of a sleeve or shoulder treatment can tweak a gown to work for a particular theme or turn an almost right dress into the dress that perfectly fit’s a bride’s vision.