<< This man is still known because he became Christian. Therefore his name was remembered and written down by those living in cloisters during the Middle-ages. >>
Sure, if a person becomes Christian and strives for good and righteousness he/she can be cannonized and remembered throughout history as well. Over 50 million Orthodox Christians know who Demetrios is and seek to be like him. Dozens of churches have been named in his honor. Not a bad legacy for a guy that simply lived out his righteous convictions, eh? We collect and track the deeds of the people who make an impact on our faith, that way we can remember them and see their lives as an example to live by.
<< If those cloisters never existed and noone would have bothered to write down his name, he would be another nameless person, like so many others of which the names and deeds they've done are forever lost in history. >>
That's why those men were formed together, to write down the names of the Saints and their deeds. These roles continue to be added to (Mtr Teresa will be another example) and numbers of saints that are regularly remembered and revered number in the thousands, simply because they chose to follow their hearts and put their faith in God.
<< Giving food to homeless or starving people won't leave a very noticable mark in history. >>
You think Mtr. Teresa will be soon forgotten? No. And neither would you if you worked as hard to take care of the poor and needy as she did.