Phones should be more flexible, so that you can modify them after the fact. A simple example of that is SD cards . When a phone supports an SD card, it simplifies the process of choosing a storage capacity, because you can always add more storage after the fact, rather than having to buy a whole new phone for a high price. Which in turn may make you buy a higher capacity than you may actually need. In fact, unless you’re buying the model that has the maximum built-in storage, you’re better off buying a phone that has an SD card slot, because you keep your options open.
Another example is the OS. For example, I wanted to try out Xiaomi’s MIUI, but didn’t want to buy a whole new phone for it. Fortunately, Xiaomi makes MIUI available for download, so that you can flash it onto your phone. Unfortunately, the process removes your existing OS, which is not okay when you’re just trying out something new — you’re not sure that it will work better than what you currently use.
One solution to this is having the OS installed on the SD card, so that you can use another SD card to install MIUI, while keeping your main Android OS on the first one.
You may be inclined to think of all this as appealing only to geeks, who are a tiny fraction of the market. I thought so, too, but then I realised that the need for change is a common human need for everyone. Which is why people keep switching phones every once in a while. What if you could do that for zero cost, rather than having to pay tens of thousands of rupees to buy a new phone?
Google’s Project Ara, which aims to build a modular smartphone, lets you swap out storage and in fact every other part of your phone. This works very well if, say, you happen to take an interest in photography some time after buying your phone, but find that it has a bad camera. Rather than having to buy another phone, what if you could just add a good camera? Or, if it has a good camera when you bought it, better camera modules may have been introduced later.
Or, if you want a top-quality camera, rather than having to spend a lot of money to buy a flagship phone, which has a lot of other stuff you may not care about, you may decide to spend far less money buying just a better camera module. This is like buying a car with a lower trim level but with the extra accessories you want, rather than being forced to spend a huge amount of money buying a higher trim level, which has a lot of stuff you may not care about.
Project Ara is ambitious and promising, but until then, a more limited version of the idea is installing the phone on the SD card.
This matters especially to unsuccessful platforms like Windows Phone. If someone sold me a Windows phone with the option of switching easily to Android if I don’t like Windows, I may be inclined to buy it. Otherwise, I wouldn’t be willing to invest tens of thousands of rupees to try out an OS I may not like. Phones that let you change between Windows and Android would also satisfy the need for change. If, after a couple years, you’re bored with your phone, you can switch OSs to give it a new lease of life.
In summary, people have a need for change, and phones that accommodate that need without forcing users to spend a lot of money buying another phone may be very successful. Change can be between variants of Android, such as stock and MIUI, or between completely different operating systems, such as Windows and Android. You should be able to try out a new OS without wiping your existing installation. And you should be able to change the storage capacity in your phone by buying an SD card. These ideas will make phones more flexible, make the buying decision easier, let users do more, and save money.
 It’s secondary what protocol or app you use to copy things back and forth to your SD card: USB mass storage, USB MTP (Android File transfer) or proprietary (iTunes). It’s also secondary whether you can eject the card from your phone and insert it directly into your laptop, or whether you must use the card only through your phone, by connecting it via a cable.