Going vegetarian doesn't always mean you're choosing the healthier option. Food manufacturers have really jumped on the opportunity to exploit the vegetarian market, and the result is a whole line of foods that are far worse for you than a bit of meat. So many veggie burgers are full of artificial flavoring, MSG-like ingredients such as disodium inosinate, and carcinogenic caramel coloring. Don't be fooled into thinking anything 'vegetarian' is healthy - you've still got to read the label and make sure it isn't going to do any harm to your body.
The great news for vegetarians is that there are plenty of healthy veggie burger options on the market (though they're probably not mass-produced and available in all supermarkets). Look for burgers that contain healthy ingredients such as chickpeas, quinoa, nuts, lentils, and actual veggies.