I've seen a lot of photos of samurai in armour from the very end of the Edo period. As I understand it Japan had been at relative peace for decades at that point, did that amour actually serve any practical purpose? if "no" why did they still have it?

It's important to remember that being a samurai wasn't simply a job that one picked up. It was very much a social caste that members were born into, able to trace their family's/clan's lineage and history as samurai, and maintained their role in society was as a caste of professional warrior vassals.

Armor and weapons of good quality were passed down as inheritances. Good armor and weapons would cost a fortune and be difficult to replace. There were also armor and weapons that were quite literally works of art, meant to be put on display and admired for their quality and price. But any equipment of decent quality would have been continued to be stored and held in the armories of samurai houses.

Japan as a whole might have been at relative peace, especially when compared to the brutal land-rending warfare of the Sengoku Period, by no means did fighting completely end. In fact, there are plenty of notable conflicts that occurred during the Edo Period and of course there was always pacification of those who rebelled/threatened to rebel, hunting down bandits & pirates, policing actions, etc.

And even when at peace, one never knew when the next war would start. We have the advantage of hindsight to know that no major wars or invasions would occur for hundreds of years but the people living at the time wouldn't know if they would be at peace tomorrow or not.

So, samurai were supposed to be ostensibly ready at all times to take up arms for their lords. Even as they added being effective bureaucrats to their skill sets, they were still supposed to be training and readying themselves for a potential outbreak of war.

And of course, armor and weapons had ritual importance as well. There were many events where a samurai might be required to come 'in uniform', whether it was a religious/spiritual ceremony, attending their lord, acting as diplomat/official, participating at tournaments, etc.

Finally, being able to own armor and weapons was a class privilege. Peasants and craftsman were, for the most part, never allowed to carry or even own weapons. That was the privilege of the samurai, which they held dear.