So, you want to be able to wear full steel plate armour, fight with a five-foot long two-handed sword, carry a backpack stuffed full of things and have a spare sword and shield just in case? Chances are your character would have been training from a very early age to be able to manage this and would have developed some serious muscle to fight effectively. While not as heavy as they seem, it still would have taken a lot of muscle to use when combined with armour and other equipment.
The Strength attribute is fundamental in other ways as well. Consider the raw strength of a Cave Troll or Ogre, massive hulking creatures able to pummel other creatures with massive fists alone. Runequest for example also takes into consideration other attributes such as Dexterity and Intelligence, (being able to land hits where you want them and being able to plan and outsmart your opponent to do this!) all this stacking up to give you a final score that will hopefully give a decent overall attack bonus.
How attributes work in relation with each other really depends on the game played. As a basic attribute Strength is one of the most important for those wanting to play a martial class, so rolling up a character and deciding their attributes can be tricky because whilst your character is as strong as an ox, chances are that some of the other attributes which shape your character may not be as exemplary. You might play a warrior who can decapitate a foe in a single blow but might struggle to do up their armour buckles due to a poor natural dexterity.