If you come to this page having googled this question, the answer is "absolutely they can!!"
Certain foods can help you lose weight and too often we get hung up on questions of calories and portion control. While it is critically important to control your calories and get an handle on portion control, you MUST eat a diet high in "Nutrient Dense" foods.
The REASON is that foods that are nutrient dense have high concentrations of vitamins and minerals in them. When your body starts to get enough nutrients, it starts to feel less hungry. Yes, you need to fill up your stomach, hopefully with foods that contain fiber. But your body is smart. If you eat a food like bread which is LOW in nutrients, your body will keep asking for more food in the HOPE that it gets the nutrients it needs.
If you ever suffer from food cravings, this is likely the reason. However, when you start to feed your body with foods that are Nutrient Dense (like Yams and Sweet Potatoes), your body will feel less hungry over time.
Which will usually help you lose weight, if you maintain good portion control.
There are foods that are even higher in nutrients than Yams and Sweet Potatoes. Based on Dr.Fuhrman's ANDI Food Scoring System (which you will see in the produce section of every Whole Foods Market in the US) Bok Choy has ten times the hunger fighting nutrients. Sweet Potatoes help fight the sweet tooth, so if you add a serving of greens, you'll really be helping yourself.