The short answer
A learning management system delivers content and records that someone completed it. A skills assessment and development platform measures whether a person can perform a skill at a defined level, and runs a plan against the distance to that level. Completion and capability are different facts, and an organisation usually needs both.
The question is normally asked as though one replaces the other. It does not. If you have an LMS and no way to verify capability, you know what training was consumed and nothing about what changed. If you verify capability with no way to deliver learning, you produce accurate gap reports and no means of closing them.
Where an LMS is the right choice and this is not
If the requirement is course delivery — hosting video, SCORM packages, structured curricula, enrolment and assignment — PDP Quest is the wrong tool. It has no course player and no content library, and building your training around it would leave you without the thing you actually needed.
The same applies to compliance. If an auditor needs evidence that every employee completed a named course by a named date, that is a completion record, and an LMS is built to produce it. An assessment result answers a different question and will not satisfy the audit.
Where an LMS runs out
Completion is a weak proxy for capability, and anyone who has run training knows it. A course finished in November tells you nothing about what someone can do in March, and it never told you much about what they could do in November either.
That is the gap this fills. Levels are defined in writing, assessments are pitched at a specific rung, and passing takes 4 correct out of 5. The result is a level you can point at in a promotion conversation, which a completion percentage has never been.
Using both
The sequence that works is measure, plan, learn, re-measure. Assess to find where capability actually sits, build development plans against the gaps that matter, use the LMS to deliver whatever closes them, then assess again to find out whether it worked.
That last step is the one most organisations skip, and it is the only one that says whether the training budget did anything. A plan completed and a level moved are different events.