These "apps" allow you to open and save the same document formats the desktops versions does compare this to opening older Microsoft DOC (etc), OOXML (DOCX etc) and other formats opening in the Word or Excel App's, there just import the document and convert that to a closed-formatted OOXML which you may be able to adjust and save, but you end up with two documents (which is rather confusing for users and has the obvious goal to lock you in the closed OOXML format). It may be the greatest for creating new "large and complex" documents (desktop app is a better fit there) but rather the ability to Open documents (-without- conversion) and make small edits (typo's, adjustments) then save back in the same format. "I don't understand why anyone would even want to use a smartphone for word processing or spreadsheet editing" The session last Fosdem on this can be found there. This LibreOffice version in these app stores needs to be paid for to cover the costs of getting the solution in these stores.
Later this year the Document Foundation (the organization behind LibreOffice) has planned to release Apps which are branded as "LibreOffice" in app stores, not limited to IOS and Android but also for MacOS and the Windows Store so installing and updating get's easy for users as it's get harder (more restrictions) to install software outside from app stores. Tough it branded it's still open source LibreOffice. Collabora has some sponsors which allowed them to develop LibreOffice for IOS and Android and also spend a load of their own time to get this done and get you these apps for free. The LibreOffice for IOS and Android Apps are today branded as "Collabora Office", reason is one has to pay (infrastructure, tools, time) to get an app in the different appstores. There's also a CIB build of LibreOffice which also provides enterprise level support (never used it but think this is branded as Libreoffice Power By CIB).
The single (most important) reason Collabora Office exists it is just another build of the LibreOffice code which enables Collabora to get you (companies, organisations) Enterprise- and Long Term Support, if that's important to you it's more like a "Powered by Colabora" LibreOffice. A fork would mean breaking and never look back, like both Apache OO and Libreoffice forked from the original project. Collabora Office is not a Fork but rater a Branch as it closely follows LibreOffice's branch while major releases are based on the LO Still (stable) and pulling patches in during the lifecycle of that version while pushing new features and patches upstream to LibreOffice.