Spanish administration has a counter and it has a website, and the website is the half that saves you mornings. What a certificate is good for, why identity data has to match exactly, and how not to lose it.
The visible part of Spanish bureaucracy is a queue. The invisible part is the sede electrónica — the online front door of nearly every administration — and it is where a surprising share of what you will need over the next few years actually lives. The credential that gets you through that door is a digital certificate, and setting one up is the single highest-return afternoon a new resident of Sevilla can spend.
A certificate is, in essence, a cryptographic proof of who you are, installed on your machine or held on a card. It is issued by a recognised certification authority after you have proven your identity, and administrations accept it in place of you standing in front of someone with a passport. There is also a credential-based alternative for many procedures, and for a lot of everyday tasks either will do; which suits you depends on how often you expect to deal with the administration and from how many devices.
What it is good for is broad and slightly unpredictable: requesting appointments, obtaining certificates about yourself, dealing with tax filings, reviewing what a public body holds on your record, handling vehicle and traffic matters, and receiving official notifications. Not every procedure is available online, and the list grows and changes, so the honest answer to "can I do X with it" is usually "check the relevant portal, because it may have changed since the last article about it".
Here is the part that causes the most failures: the certificate binds to your identity data exactly as the administration holds it. Your name, in the order the register has it, and your number, character for character. Passports with middle names, two surnames, accented characters or a transliteration from another alphabet are classic sources of mismatch. Before you begin, confirm which identity document and additional reference, if any, the chosen issuing authority currently asks for, then use precisely the recorded spelling throughout.
The issuing process generally follows a pattern rather than a single fixed set of clicks: you make a request online and are given a code, you present yourself somewhere to have your identity verified, and then you return online to download and install the certificate. The detail that trips people is that the request and the download often have to happen on the same machine and browser. Follow the issuing authority's current instructions rather than a blog post — including this one — because the technical steps are updated regularly.
Once installed, treat it like a key rather than a file. Export a password-protected backup and store it somewhere you control, do not install it on a shared or work computer you might lose access to, and know how to have it revoked if a laptop is stolen. Certificates also expire, and renewal is much easier before that date than after it, so put the expiry in whatever calendar you actually look at.
A closing caution worth repeating: a certificate proves your identity online. It does not alter your residence status, does not create or remove any tax obligation, and does not make a procedure available that is not otherwise available to you. It simply means that from a desk in Sevilla, on a Tuesday evening, you can do things that would otherwise cost you a morning and an appointment. Where your circumstances are unusual, ask the issuing authority or a qualified adviser rather than guessing.