By having a choice, it gives your children some sense of control as you try to get them to do what you want.
The key is offering only limited choices. So don't simply ask your kids what they want for lunch. Instead, ask if they would like a sandwich or a hot dog. Younger children can especially be overwhelmed when they have too many choices.
But you also don't want to offer a choice when no choices exist. For example, don't ask them if they 'want' to brush their teeth now, take a bath, or get ready for school. By asking them, you are essentially offering them a choice between doing it or not.
In these circumstances, it is usually better to simply say something like 'it is time to take your bath.' You can still offer some choices though, like by saying 'it is time to take your bath, what toys do you want to bring into the bath with you tonight?'
When there is only one choice, you can still try to offer your child a 'choice.' For example, if your child doesn't want to wear a helmet while riding his bike, you can offer a choice of wearing the helmet or simply not riding his bike. Or if he doesn't want to sit in a car seat, you can ask if he wants to sit in his car seat or stay home all day. Having this kind of choice also helps to give your younger child a 'way out,' instead of having a tantrum or making you give in and do the wrong thing.