In 2 Peter 1, the apostle is recommending growth in holiness.  A string of positive characteristics is presented in verses 5-8:

• Supplement faith with virtue
• Supplement virtue with knowledge
• Supplement knowledge with self-control
• Supplement self-control with steadfastness
• Supplement steadfastness with godliness
• Supplement godliness with brotherly affection
• Supplement brotherly affection with love

Why? Because if these qualities are increasing, they keep you from being ineffective or unfruitful.