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Well foreach basically does the following for you:

pawn Код:

foreach(Player, i)
{

is the same as:

pawn Код:

for(new i = 0; i < MAX_PLAYERS; i++)
{
    if(IsPlayerConnected(i))
    {

It's quicker to use foreach, and much more efficient, hence its wide usage throughout sa-mp servers. There's a full thread explaining its uses. Also, I would have to say that creating a loop within OnPlayerUpdate for EVERY player connected, would have to cause much more usage when compared to using the simple 'playerid'. That's general knowledge I am speaking of, although I have no sufficient evidence to actually back up my claim such as how usage is generated etc.