In life we all go through different experiences which teach us different things. We will also make various mistakes which can be a source of further learning but every now and again we may get support from others which helps us avoid those mistakes. While one should not court advice from all and sundry, the one who refuses to heed counsel at all may end up counseling themselves and is hence limited in what they can learn or teach themselves. Self-counsel tends to be very kind or too strong while another person can give a more balanced view or one pregnant with the kind of experience and/or expertise that is needed. So with this proverb our elders were stressing the importance of listening to advice from others in order to escape foolishness.
The proverb is used to advise on the importance of listening to other people’s counsel and to ensure one always has people who can advise them. One who teaches oneself has a fool as their master.