Would a person get in reality a bonus to OB on a horse or on a table
or
do we all believe it, because we seen it on TV
No it isn't "just from TV", it is a Tactical fact, which is Why Mounted cavalry were so popular in certain cultures. A Mounted combatant had an advantage over an unmounted Opponent. It's Show in TV (even if it might be more exaggerated) becuase it is a reality. This is also Why Generals would seek to have there troops positioned on Higher ground. It always sucked to be the Guys who "Didn't" get to pick the terrain. it had more effect on the Missile than the melee, but even in melee it was of importance for several factors.
#1 the enemy had to Come up hill at you thus tiring them out more.
#2 Less likely to hit you in a vital area becuase they are lower,
#3 if using shields, especially Body/Tower/Full shields (what ever you want to call them) it became easier to keep the opponent away from you when they are lower down.
#4 you ability to hit them in the head or chest became easier as they can't jump over or dodge as easily.
#5 even if they are using a shield, they are more exposed. keeping the shield at normal level, their heads are more easily exposed. raising the shield to protect the head Tired the opponent out more (try holding a bicycle Tire, spokes and rim, over your head and Swing a broom at the same time) Plus By moving it higher you exposed yourself in the lower regions.
#6 It's much easier to dodge a blow aimed at your legs (legs being one of the most nimble parts of the body) by side stepping or jumping, than it is to your mid section.
#7 Much easier to knock a guy down if your up higher than he is. On a hill, pushing you opponent back with your shield often results in them tumbling downwards, In the other Direction, it is harder to keep you balance on while trying to attack a guy higher up the hill than you.
I could go on. But I think that should be enough to explain WHY being on higher ground is an advantage.