Certification is not mandatory but is certainly helpful and may be the difference between you and other candidates for a job. I find recruiters are not only looking for certifications but they are starting to ask and look to see what Trailhead modules you have completed too so they can see that you have made a commitment to ongoing professional development. If you got your certification 3 years ago and have done nothing since then your skills are getting out of date and you are not showing initiative or a desire to learn and improve yourself. With 3 new Salesforce releases every year you can't let those skills go rusty.
Volunteering is another way that you can demonstrate your work experience/readiness. If you can present at an interview and talk about how you helped an organisation implement Salesforce and what business problems you helped solve (even if only a minor part of the implementation) then will also help you stand out from the crowd.
Need help getting started on your Admin Certification? CertifyCRM.com offers a FREE online course to prepare you for your Certified Salesforce Administrator exam. (and we include lots of Trailhead badges in the course too!). Got your Admin cert? We have a paid course ($30) for Certified App Builder too.