It is different depending on whether you are young or old. Young people often change quite a bit between 18 and 26. Waiting allows you to learn to stand on your own and discover what values are most dear without replacing mom and dad with a spouse. Also, it gives you time to save for a wedding, finish your schooling, etc. It avoids infatuation marriages and cements that commitment. For older people it helps you avoid getting married quickly to the wrong person because you are lonely i.e. divorce or death of spouse and helps you avoid people who are just after your money at a vulnerable time in your life. If the commitment is truly there, the wait won't kill it off.
It's better to have a long engagement and learn how each other copes with the different things that happen: financial, future in-laws, health issues, work stress, etc; then a quick wedding a few months later.
The first thing for a woman to consider is......is he getting "engaged" just so he can have sex with you. Make it clear that there will be no sex until marriage. Then you will see if his intentions are serious. Just because he "puts a ring on it" doesn't mean that you will jump right into bed. Also, a good argument can bring out the true character of a person. If you have nothing but "good" times, you won't know the real person you are considering spending your life with.