Well...that is a marinade to make a less-than-great cut of meat taste pretty good.
The soy sauce impregnates the fibre of the meat to infuse it with salty taste (because a lesser cut doesn't have much flavor.)
The brown sugar is an extreme way to 'encase' the meat so that it doesn't 'drip out' all the juice. The pineapple juice will also help with that....
Vinegar?? That's a surprise...although, it might link-up, flavor-wise, with the Sherry, but I tend to doubt the necessity of both the Sherry AND the vinegar. Throw an old, half-drunk bottle of red wine in. I suspect THAT was the original recipe.
Garlic? ABSOLUTELY!!
You know what? I'll bet this would be a pretty decent marinade for chicken. I wouldn't use this on anything but the most economical cuts of beef....
But on
chicken..... I say, add some fresh ginger, and more garlic, and .....
ohh baby...