Let's Encrypt offers free TLS certificates, including wildcard certificates. Is there ever a reason to pay for a certificate? Is it just "we have to pay for everything so we can sue someone if something breaks" corporate policies?
The commercial certificate issuers provide technical support. So in case something goes wrong, you have access to a support contact instead of hoping for someone from the community to answer you. Commercial providers issue certificates for longer periods than the 90 days LetsEncrypt certificates.
Together with this there are perks like warranties, proprietary certificate management tools and fancy "this site is secure" .jpgs / site seals.
I’ll throw in another reason for using other CA’s than Let’s Encrypt: diversity. You may not want your entire infrastructure to depend on one single CA. Using certificates from different CA’s reduces the impact of CA issues (compromise, OCSP problems, etc). It also allows you to have some ‘backup’ certificates signed by other CA’s ready in case of issues with the main certificate or its issuer. I know at least a number of banks that do this.