An SSL certificate encrypts the communication with the services, which takes place through the use of your domain. This increases security by preventing the data from being "read" by the vastness of the Internet. Furthermore, it confirms the identity of the domain owner, which inspires a high degree of trust in the visitor. Legally, it is regulated in the Federal Republic of Germany that if you process data of your visitors electronically (this also includes a contact form), you need an SSL certificate and thus ensure secure communication channels.
Please also note the new regulations that have resulted from the introduction of the DSGVO: https://dsgvo-gesetz.de.