Reasons you may want to charge On-the-Go

[Bicycle365]

Frequent charging is usually a bad thing in terms of battery life but there are those folks who want the convenience of being able to top off .

Reasons on-the-go charging may be needed

Smaller Battery Pack

Not everyone can, or wants to, buy the largest battery available. Weight and cost are the two primary penalties you pay for larger battery packs. If you can minimize this by charging on the go then it is worth looking into. If 95% of your trips require only 10Ah, spending a lot of extra money on a 20Ah pack does not make much sense. The only benefit to doing that is you can safely charge at a higher amperage since it would be a lower C-rate than a smaller pack.
A good example is if you commute by ebike and a one way trip uses 4Ah, then hauling a 20Ah battery around is not necessary or cost effective. In this example a 6Ah pack would be sufficient if you can charge during the workday.
Another example is recreational trail riders who drive to a desirable trail head and periodically return to their vehicles for breaks. The ability to charge packs from a 12v system would allow unlimited rotation of several packs throughout the day.

Travel

Bicycle touring is a fantastic way to travel, be it a day trip or an extended tour. You get to experience your surroundings at a human speed without wizzing by in a rush to "get there". Ebikes have begun to open this up to a much larger group of cyclists with the increased range and capacity of the newer battery packs. It is entirely possible to cover the mileage that traditional cycle tourist cover by ebike if you use multiple packs or can charge on the go.
An example of this is a S24O (sub 24 hour overnight) trip where you ride to a destination and return the next day. By using an on-the-go charging approach you can ride one (your battery range) in the morning then enjoy a leisurely lunch, museum visit, nap, hike etc... while you recharge and then continue to your destination in the afternoon.

Range Anxiety Relief

Range anxiety is a part of owning an ebike unless you know you have far more capacity than you ever ride (a rare occurrence). On-the-go charging buys you some relief from the worry by knowing that if you extend your planned ride you will not be walking home. The ability to recharge even a small portion of your capacity anytime you want can go a long way to making the experience more stress free.

[joeaverage]

Something to consider is that if you have a tiny battery and your typical trips consume a whole charge, you might be better off with a bigger battery just so you don't deep cycle it so much each time you ride. Might help the battery age better. I charge at work and not at home. I don't dwell anywhere else long enough to make on-the-go charging worthwhile unless I charged at a very high charge rate which can be hard on the battery.

Some folks have different attitudes about batteries. One might wear the battery out quickly until it no longer provides the range and then replace it. Might last a year or so. Their budget can handle it. They are offsetting the cost of an automobile in an urban environment so they are money ahead compared to suburbanites who have a car and an ebike.

Other people like me may prefer to be gentle with the battery and only use 35% of a charge under normal circumstances and charge slowly so the battery might last 5 years.

Battery prices are coming down but not that much. Mostly battery capacities are going up and the price is remaining near constant. That's probably where the demand is. A 36V 8AH battery perhaps just isn't in great demand. Due to typical life expenses I prefer to make things last as long as possible so my hobbies don't put a burden on the family budget - but that's just me.