Shares provide a scope for capital gains along with a stake in the company's decisions, though they are risky.
Debentures, on the flip, provide the investor with a safety margin - regular gains, but the debentureholders do not have any say in the company's affairs.
So, an investor will invest partly in shares and partly in debentures of a company as it would provide him with safety of investment, with regular savings, alongwith providing him with the opportunity of occassional capital gains.