The time value of money is very important in understanding this issue. Money today is worth more than money next year, two years from now, etc. It's a well understood economics concept, and well worth reading about if you have some, well, time.
Not only is money literally worth more now than later due to inflation, but there is the simple fact that, assuming you have money for the purpose of doing something, being able to do that thing today is better than doing that same thing tomorrow. "A bird in the hand is worth two in the bush" gets to this rather directly; having it now is better than probably having it later. Would you rather have a nice meal tonight, or eat beans and rice tonight and then have the same nice meal next year?
That's why interest exists, in part: you're offered some money now, for more money later; or in the case of buying a bond, you're offered more money later for some money now. The fact that people have different discount rates for money later is why the loan market can exist: people with more money than they can use now have a lower discount for future money than people who really need money right now (to buy a house, to pay their rent, whatever).
So when choosing to buy a bond, you look at the money you're going to get, both over the short term (the coupon rate) and the long term (the face value), and you consider whether $80 now is worth $100 in 20 years, plus $2 per year. For some people it is - for some people it isn't, and that's why the price is as it is ($80). Odds are if you have a few thousand USD, you're probably not going to be interested in this - or if you have a very long term outlook; there are better ways to make money over that long term. But, if you're a bank needing a secure investment that won't lose value, or a trust that needs high stability, you might be willing to take that deal.