I would say the reason I paid over 1k for my past two bags (
Gucci) was due to my father's education. He would love shopping in the finest retail stores so I followed suit. I went into Gucci and fell in love with their thick fine leathers. One of my bags, a grey one has taken on a gorgeous "aged" look and I love it more than the day I bought it. I recently wore it on a trip with my kids. The bag is now going on two years? I haven't received a compliment on it from strangers but another Gucci customer would recognize it in a nano second. It's a classic and the fact that it does not attract any attention, increases my love for it!
I really feel people must buy within their limits and try and not be swayed by what others are buying but what they can afford. This bag became affordable to me when it was reduced 30% at the boutique and I knew I would enjoy wearing it for many years.
I guess the bottom line answer to your question should be because one can afford it or has saved up for it. My definition of affording it is having the cash to just buy the bag on the spot. Whether it's a $1k+ bag or a $300+ bag it should be something the wearer loves and enjoys!