Very quickly after you become a parent, you realize just how wonderful it is to know when you are tired and lay down and fall asleep. Children struggle to notice their own cues and often fight off sleep with all that they have for one reason or another. My husband (and many other adults) has a hard time falling asleep if he hasn't given himself enough time to wind down at night before he tries to go to bed. That is exactly why bedtime routines are a tried and true practice worth implementing in your home!
When my daughter was an infant, she had a serious case of acid reflux and a very sensitive digestive system. Evenings were the worst time for her, and us as well because we just wanted her to feel better. We quickly discovered that warm baths and infant massages helped her immensely. So she sat in a bath for quite a while in the evenings, we did the massage right after, I nursed her, swaddled her, and then laid her down for the night. Without really thinking intently about it, we implemented a bedtime routine. One day it occurred to me that she did, in fact, have a routine, and we haven't quit since then.
The specifics of the routine have changed a bit here and there as she has gotten older, but one thing that has not changed is the bath. She loves them, and she knows when bath time comes that bedtime is going to follow quickly thereafter. Does that mean that she never protests going to bed? Definitely not. But, it is not a surprise to her which helps because she knows no matter what she says, it is happening.
So what is the big deal with these bedtime routines? Well, routine and familiarity. Children thrive when there is familiarity and routine in their life. Let us use the actual bedtime as an example here. Children of all ages (and adults for that matter) experience natural cues when tired. As adults, we are aware of those cues and what they mean. Young children do not know what they mean, so they have no clue why they are feeling grumpy or slowed down etc. As parents, we can pick up on those cues in our kids and can build a bedtime routine around them. 
By providing a routine that accompanies the tired feelings they are experiencing, the whole experience will slowly become familiar. After implementing your routine for a while they will begin to understand that their routine and bedtime go hand in hand with the tired feelings they experience at the end of the day (and nap time too) are followed by laying down to go to sleep. We are at the point now that we can talk to our daughter and say, "You are feeling sad/grumpy/etc. because your body is tired and trying to tell you it wants to go to sleep. So, it is bedtime and when you wake up you will feel so much better!"
We have a couple books that talk about going to bed, and the routine followed before going to bed. These seem to help our girl be ready to accept the fact that bedtime isn't the worst, because her book's characters are all laying down to sleep too! Slumberkins books are wonderful options (and even address other hard topics for kids.) She also has multiple stuffed animals and one blanket that she sleeps with every night. She always has her blanket right up by her cheek, and her little-stuffed friends arranged just so around her. I never try to change or mess with her set up because she has obviously found her own little routine and familiar space to sleep that helps her feel safe and relaxed!