Couples nowadays are more diverse than ever. Some of these couples are remarried. Some have kids from a prior marriage, and some have no kids. Some choose never to marry.
The usual common element in these relationships is the conflict that exists in the relationship. Typically one of them has a conflict between caring about their current partner (or spouse), along with any children they share, and caring about a child or children from a previous relationship (or marriage).
In planning their estates, the common element is that other than caring for their current partner, each partner has a different distribution plan (i.e., after the surviving partner passes, each partner want his/her remaining assets to go to different individuals and or charities.)
For example, the couple without kids who each has his/her own retirement account
Consequently, the couple may want to design a plan that includes a joint pour over trust. In their joint trust they would hold their assets they own together, and at the first death, each partner’s share of the trust would pour over into each partner’s own individual trust. The funds in that individual trust would then be available to the surviving partner, and if the funds are not exhausted they would eventually be distributed according to the terms of the individual trust.