There is no spectacular reason to visit the city. Well, its a main gate to most of the places in Cambodia so you kind of have to do it, plus if your there you can stop for a day to visit the killing fields and Tuol Sleng torture centre.
But the recent history seems to be forgotten pretty quick in Cambodia... which is sad cause just 30 years ago a mad leader decided to transform Vietnam in a Agrarian utopia and mass killed most of the educated people (even people that were wearing glasses cause they looked intellectual) and evacuate the cities and send the population to work in the jungle.
Further even, The Khmer Rouge (the only leading party which was actually an army) divided the population into 2 groups: the peasants were called the ancients while the population evacuated from the cities were the new people (communism cu caste. thats was something new!!!). Hospitals were not existent and if they were they were just the last place before you died, the food was rationalized and they abolish any form of currency. Their plan of reshaping the country was for 5 year and after 4 years and several months, the Vietnamese got pissed by the repetitive attacks on their border (Mekong Delta) and invaded the country.
A third of the population died. That means 7.5 mil. people died in less then 5 year.
Killing fields is a memorial place, in the center of it a pagoda was build.
Tuol Sleng torture center was before Khmer Rouge a school, and was transform during the lethal regime into a torture center. Now-a-days is a museum that lacks founding. The government interrupted financial support several years back... And so the museums its quite... empty and doesn't really help to understand the horror that took place within those buildings. Which is a pity, because people tend to erase from the memory the drama if they are not faced again with the documented horror.
While in Vietnam I've read a novel called "Stay alive, my son" which help me to understand the proportion and the impact of the Khmer Rouge regime. Its a biography of that particular time by Pin Yathay that survived during this time and succeeded to cross the border in Thailand. His children, wife, parents, uncles, aunts, cousins, all died.
A bit more than 30 years had past and Phnom Penh is as any normal asian poor city. Buzzing with people, having some nice monuments, some really expensive cars and loads of tuktuks. And, to just not forget, beggards. And even if in the first couple of hours your heart softens at the sight of a 6 year old carrying a 3 year old brother asking for " a dolla' ", dont worry. Will wear off quikly as more and more would come, not only begging but selling cheap copies of books, bracelets, postcards (10 postcards for one dolla'. They can even count them. In 10 languages!!! I can not count in 10 languagges!!!). It is a sad view but the only atitude you can have is a passive one. Don't support the "industry" or it will further develop. In the best case give them food or candies. Wait, maybe not candies cause they will tell you they are bad for their teeth... and that they actually prefer the dolla'.
But besides, in the evening you are only so lucky if you get a glimps of an elephant walking through the traffic.