The term "Multi-App" is a coined expression, same as for the ubiquitous term "App" and this Site explains the difference between the two.
The development path of the Multi-App was simply because of "necessity is the mother of invention" matters. The necessity in this case was HELPFUL information for planning a boating/walking/driving holiday in the Upper Thames/Cotswolds area of the UK. One would expect that the "Information Super Highway" in its 20th year in 2015 would cover that need most adequately.
Unfortunately what I DID find was total confusion by the I.T. Industry typified by the "keeper" of the Thames (The British Marine Federation, BMF) who HAD produced an excellent "Cruising Guide to the River Thames" in booklet form, which became "digital/printable" by way of a downloadable PDF file, so was already quite cumbersome to view on a computer even before the need to be "mobile friendly" was cast upon us by The Google Test of April 2015.
While most entities spent the money over the next two years to "shoehorn" their wide-body CSS behemoth site (which had already cost them an arm and a leg) into the "Mobile Friendly Google Testbed" by way of RSD etc, BMF decided to discontinue their PDF version and simply have a new "Explore Thames App", naturally using the customary "You Are Here!" zoomable GPS powered map. In other words NOTHING for those using computers (or booklets) and no trip planning facility even if one used Explore Thames on their mobile device.
I therefore "did my own thing" hence the evolution of the Multi-App. And because I was not encumbered by having to "bend" any existing sites, I designed it from scratch to be mobile super friendly while also working on my computers. But the mystery still remained as to why the I.T. Industry had not done the same.
Then the penny dropped as I realised there was a second part to the proverb which says "If necessity is the mother of invention, then laziness is the father". Further Googling revealed an explanation for "laziness" by Bill Gates who said he would always "hire a lazy person to do a difficult job because a lazy person will find an easy way to do it".
Finally, all that proverbial wisdom hinges on the meaning of "easy" and Bill Gates was quoting from the point of view of Microsoft and not of the end user. So the Explore Thames solution was easy to PRODUCE as it simply latches on to an existing GPS mapping system, but very fiddly to USE, especially for older folk.
On the other hand my Multi-Apps are very easy to use but very time consuming to produce, with the result that I have jumped all over the place to discover what are good "applications for a Multi-App" and what are not, and then moved to the next before completing the last. Also of course I am perfecting the ideas and technical aspects as I go, the most exciting addition in 2017 being the embedment of Google Trekker (and normal StreetView) panoramas where available/applicable.
UPDATE May 2017 - BMF have "upgraded" Explore Thames to a "guide" and it DOES work on Windows 10 and although it DOES look a lot like my Thames 'n All, it is very limited for Thames Path.
UPDATE Dec 2017 - BMF have again changed their Thames Path "coverage" from simply deferring to the official National Paths website (alas, it was not mobile friendly) to a roughly drawn Black Line 184 miles long and no explanation.