I know that Provararis use button tops as part of their bespoke solution, but the vendors dindn't make button tops specifically for them. What I meant was why do manufacturers put buttons on some cells and none of others?
Battery manufacturers like LG, Samsung, and Sony (South Korea and Japan) never put button tops on their batteries. If you find batteries from these manufacturers having button tops, they have been added after leaving the factory. Some Chinese manufacturers may make button tops because of the market that they sell to.
Button tops can help the user from inadvertantly putting a battery in backwards (reverse polarity). It's often difficult to recognize the positive end from the negative end in a flat top battery.
When connecting batteries in series (i.e. equipment that needs two or more batteries oriented end to end) flat top batteries can also be a problem. Flat top batteries were not designed to be used in series, but were designed to be used only in a battery pack. (see second link below)
Also, some battery devices require button tops to fit better, such as some flashlights.
Here are a couple of older articles on button top vs flat top batteries from several years ago, so keep in mind that some of the information may be outdated.
Flat Topped vs. Button-Topped, what's the deal?
Battery, button or flat top