1. Snail mail is back in vogue.  Clients used to get lots.  Now it's mostly bills and junk mail.  The volume decreased while their email box filled with ads and they got lots of social media posts.  Personal letters and cards sent by surface mail now stand out.  According to Hallmark, 1.3 billion cards are sent annually.  (Lets assume that includes other countries).
2. Celebrities do it.  In a few weeks the major magazines will start talking about holiday card designs chose by celebrities for 2021.  The British Royal Family and major politicians also send holiday cards.
3. The envelope is personalized.  You get letters that look untouched by human hands.  The envelope is a good clue, especially if it has a postal machine imprint.  Your cards will be hand addressed.  You will choose a great holiday stamp too.
4. You can write your own message inside the card.  Cards sent by businesses often are generic.  There's no salutation or handwritten signature.  Your cards will be different.  You will inscribe each to the recipient.  All your team members will sign it.  This shows how many people touch their relationship.  You might thank them for ten years of their trust as a client. 
5. They can see you have a family.  If you go the photo card route, the recipient sees you have a spouse and family.  Maybe a pet too.  They see you less as a business liaison and more as a family person like themselves.  It's bonding.
6. It reminds your client who you are.  Your client might know they have a business relationship with your firm.  Perhaps it's where they buy their health insurance.  They don't know they have a specific agent or advisor handling the relationship.  By reaching out, you are telling them who you are.  They might call with questions or future business. 
7. Holiday cards have staying power.  Emails get deleted.  However, holiday cards are set on a table or fireplace mantle.  They may be strung on a long cord with other cards as part of their holiday decorations.  They pass your card several times a day. 
8. You might receive cards in return.  It's a reason to get in touch.  Your client might say: "If she sent a card, I better send one back."  Their card arrives.  It's a reason for you to call and thank them.
9. You might be invited to their holiday party.  The card-in-return concept might blossom into something much bigger.  You might call to thank them for their card and get invited to their holiday party.  Now you are meeting their spouse, family and friends! 
10. Respect for the relationship.  Here's the most important message.  You are thanking them for their business and saying you value their business.  You are highlighting it's a relationship, not a transaction.  This strengthens the relationship.

There are many reasons to send holiday cards in December.  If you buy into this idea, you need to get moving quickly!