To expand on Diegocorry's comment, politeness and respect is a big thing in the Philippines. In the Philippines, there are a number of circumstances where use "respectful" language to show respect. Examples are when a person is significantly older than you, superiors (e.g. your boss), or even when customer service people are talking to you.
In regards to your question, this means all singular pronouns can turn into plural pronouns to show respect. It's simply not limited to just "kayo". For example, ka -> kayo, mo -> ninyo, siya -> sila etc.
But in which specific situations might you turn a singular pronoun into a plural pronoun to show respect? After all, when talking to your "ate" or "kuya", there is no need to turn singular pronouns into plural pronouns. That might be too polite.
There are no hard rules here, but as a general rule I think asking yourself the following question will do the trick:
"Might an average Filipino think using 'po' here is appropriate in this circumstance to show respect?"
If the answer is yes, then one I think should choose to turn the singular pronoun into a plural pronoun.
You might be also be familiar with the other phrases Tagalog uses to show respect (such as "ate", "tita", "lolo" etc), but if you aren't a quick google search will do the trick.