While the reasoning behind the Samsung designed SoC is not from a consumer's standpoint there is still a logic to it:
•when it comes to mid-low ramge phones the popular choice of SoC is a Qualcomm one, which means samsung has to get a hold of them, this includes signing a deal to buy an x amount that will suffice for the estimated demand, pay for the shipping cost, engineer the device with integration of the SoC in mind (which means the engineers need to study about it from the source).
All of that for a device that is not centered around performance or battery life...
But when using a SoC that made by themselves they can tailor the chip to the exact needs of the device, cut on (most) of the shipping cost's because it is made domestically, have the exact amount that they think will be demanded and the ability to make more if demand exceeds, make the most out of the chip because they know it at its lowest level and over all cut the cost for the end consumer.
•they can use the experience they gain from designing a chip to develop SoC for other devices such as smart TVs (a market already dominated by Samsung), IoT devices (a field still developing) and networking equipment (a field that at the high end is dominated by american companies and by Chinese ones at the low end) and make better products in those fields.
•the more companies joining the mobile SoC market (which is dull) the more competition Qualcomm has which drives them to make better products.