Do i need to have an ISO for quicker O/S Full install  

New to Win10.
Original OEM install disc was Ver 1903
Running 64 bit Pro with current latest patch update 1909.

Question is , after several future MS major updates , if i was to have to do a full recovery install at a later date ,my original Ver 1903 would need to be fully updated with updated downloads to reach current .(heaps of prior updates)

Is that the reason for having Current MS ISO , to save time during new full instalation?????.
Yes im slow @68yrs.

I wouldn't be highly concerned about it.

If you install an older version of Win 10, you will soon enough be subjected to Windows Update and receive the newest version.

Maybe you don't want to have to go through the update process? It isn't flawless, but works pretty well in my experience.

me personally never did the upgrade way from windows 7 I just backed my stuff up and did a fresh install as I usually do a fresh install every couple years.

It is no longer necessary to install the Windows version that came with the computer with Windows 10.

With a digital license on Microsoft servers you can modify the operating system or replace the drive.

A windows 10 iso is free.
Most windows 10 iso versions are available in case you want to try an older version.

Windows 10 iso (any version) can be used when troubleshooting as they have Windows Recovery Environment (RE).
The Windows RE has options for startup repair, system restore, command prompt, custom install, clean install, etc.

Making free backup images reduces the need to use Windows 10 iso for troubleshooting.