Is having a baby shower imperative? Well, no not technically, but it is important. Having a baby shower signifies the beginning of a new life, for you and your baby. If you or someone you know is having a baby, you should make sure a baby shower is in plan for the future.
Many expecting mothers don't want to “burden” their friends with having to host a baby shower in their honor, but the reality is throwing a baby shower shouldn't (and isn't) a burden, it should be an honor to throw a pregnant woman a shower, whether it be a first child, or 5th, the party will always be something that should be cherished.
The purpose of a baby shower is of course so the expecting mother can receive gifts (which is reason enough for the celebration) but also, having a shower hosted in your name is really something special that every pregnant woman needs. The depression that pregnancy can sometimes cause (because your body is changing, because you're hormonal, whatever the reason) can be significantly reduced when there is one day that is specifically designated to the pregnant woman.
As stated above, gifts are probably the main reason for hosting a “party” for the new mommy, and the reasons can differ slightly as to why this is important. Many new mothers don't have everything they need for the arrival for the new bundle of joy, or they don't have the money to get everything, or they just don't know exactly what they need, so the gifts they receive help immensely with the preparation for the new arrival.
There is no set rule as to whether or not there should be a baby shower for the “second” (or third or fourth etc.) pregnancy. This is entirely up to you and the hostess of the party. Keep in mind, some woman have a girl first, and then they become pregnant with a boy, or some people don't have another child until 5-10 years after the first… either instance, chances are, the mother still doesn't have what she needs for her new baby. A baby shower is perfectly acceptable for any number of pregnancies.