Because the paperwork is heavy and dealing with the immigration office is not straightforward, plenty of people hand the whole thing to an agency. An agency is not automatically the better choice, though. Splitting cases into those where the documents are fixed and those where judgement is required makes the decision much clearer.
What doing it yourself leaves you with
The obvious gain is that you pay only the statutory fee. The less obvious one matters more over time: going through it once teaches you what your own residence status actually requires, so the next extension or change of circumstances is something you can assess yourself instead of paying to be told. The costs are time and stress. If your Korean is still developing, reading the guidance and making enquiries is work in itself, and a missing document means another visit and a delayed timeline.
When an agency earns its fee
Agencies have processed the same case types repeatedly, so they identify the required documents immediately and can handle translation and notarisation in the same pipeline. Where a case needs several documents issued in your home country, that coordination saves real time. Against this sits the fee: a straightforward extension can run to several hundred thousand won, and complex changes such as marriage or investment related cases sometimes exceed a million. Pricing varies between providers, so confirm the scope and what is included before signing anything.
How to tell which case you are
If the document list is defined and nothing about your situation is unusual, doing it yourself is the sensible route. A student extension that comes down to an enrolment certificate, a transcript and proof of funds falls in this category. Where an agency genuinely helps is the other side: changing your status entirely, cases where it is unclear whether you meet the requirements, or any history of a previous immigration problem. Those involve judgement rather than assembly, and judgement is what you are paying for.
The phrase that should end the conversation
Avoid anyone who guarantees your visa will be approved. The decision belongs to the immigration authority, and no private company can promise an outcome. Cases exist of large fees taken upfront followed by failure and no refund. Get the terms and the refund conditions in writing, and check reviews and reputation before committing. Free options exist too: HiKorea publishes the procedures and document lists, and the 1345 immigration contact centre advises in multiple languages. A single call there often settles whether your case needs an agency at all.
A sense of cost by case type and how to check an agency's credentials are in the visa agency comparison on the WeBring blog.