The short answer, is so that we can "future-proof" the bike. 

The more detailed answer, is as below.

The Titan is an E-bike intended and designed for long-term usage - we're talking decades here. Conversion kits, enable the Titan to accept new and/or improved e-bike battery and motor technologies as they come along. Integrated e-motor and battery systems, will not.
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When a bike with an integrated motor and/or integrated battery gets older, the motor and battery get harder to repair as the parts become more difficult and more expensive to source. This is due to the manufacturer of said motor or battery, gradually withdrawing their support for those particular products, because they (the manufacturer) want to focus on the newest versions or models. So eventually, that motor and battery system, becomes obsolete. And when this happens, the WHOLE BIKE becomes obsolete, not just the motor and battery... What a terrible waste, because the rest of that bike is usually still perfectly fine. To make matters worse, due to the incredible speed of e-bike technological development, an integrated e-bike made today in 2025, could easily be obsolete by 2030!
 
Lots of old integrated e-bikes inhabit many a scrapyard for the above reasons. By using a conversion kit, such as the range of e-motors offered by Bafang - [the BBS01 is fitted to the Titan], if a particular motor or battery becomes difficult to repair, because the parts have become hard to source or even obsolete, all the owner has to do, is simply CHANGE OUT THE OLD SYSTEM for a NEWER ONE, and thus, doesn't need to throw away the whole bike!

Thereby, prolonging the life-cycle (pun very much intended) and usability of the WHOLE BIKE.
 
Likewise, if the owner simply wants to use a different brand of e-motor and battery system, such as Tongsheng or CYC as examples, then, again, all they have to do, is CHANGE OUT the motor and/or battery.

With integrated motors and/or battery systems, this is not possible. For example, you cannot switch a Bosch integrated motor for a Yamaha integrated motor, because the space around the bottom bracket position for their respective motors, the mountings and attachments, are all deliberately made to be very different to each other... thereby, forcing the bicycle frame manufacturer to only be able to use that particular e-motor manufacturer's product, and, to make swapping out for another brand, impossible. 
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At the time of writing this, the development of Solid State Batteries for actual use is becoming more and more of a reality, and, are very much on the cards for use on E-bikes. Also on the cards, are powerful new E-Motors that can be made small enough to be placed within a Tandem-bike-sized front bottom bracket. Current E-bikes fitted with integrated e-motor and/or integrated battery systems, cannot accommodate these coming new developments, so consequently, again, the whole bike, not just the motor and battery, will have to be scrapped!

If you think this is being alarmist... just remember that virtually none of the original E-bikes from the early 2000s which had integrated motors with integrated Lead-acid, nickel-cadmium (NiCd), or nickel-metal hydride (NiMH) batteries, are still in use today.
 
So, to reiterate, by using conversion kits, the Titan is "future-proofed" to allow for forthcoming e-motor and e-battery developments, thus enabling it to remain relevant and sustainable for many many generations to come. If you like, it's the "Dutch bike" for the 21st Century.