I am a cradle Catholic/lapsed Catholic who, as an adult chose the Catholic church for herself. I went to church regularly as a child and sporadically as a young adult. Along the way, I was baptized, catechized, confirmed and received the sacrament of Reconciliation (confession, in casual parlance). I was married outside the church, then later in it, a perfect demonstration of the fact that there were details about my faith whose significance I didn't fully understand until they became relevant to me.
Mass intentions are one of those things. When my mom passed away last summer, many of our Catholic friends arranged to have Masses said for her. Though I was aware of this practice, I never really "got" it; yet, when people took the trouble to do this for my mom, I felt comforted in a way I hadn't expected.
All of these kind actions on the part of friends and family prompted me to do the same, and I swallowed my "I should know this" pride and asked the necessary people how to set up a Mass in honor of my mother at my own parish. The Mass at my parish is in a few weeks and today, reading email, I discovered an email from a friend whose parish was saying a Mass in my mom's honor today.
Why do Catholics do this? Aside from the comfort it offers to survivors, it's a way of honoring the living, remembering the dead and offering something much larger than our own simple prayers for those we wish to remember.
Want a better explanation? I did, so I went looking and found this. There are lots of more complex theological explanations available, many from more credible or scholarly sources, but this one stuck (mostly) to layman's terms and answered the questions I had.
In my opinion, there's a special kind of comfort to be found in this sort of commemoration. Once again, this is something I didn't fully understand until it was relevant to me. I will always be grateful to friends who reached out to my family and remembered my mom in this way and, as a result, I know it will be part of the way in which I offer comfort to others in the future.