Drainage is one of the most common post-purchase shocks precisely because nobody looks at it before exchange. A full excavation and re-lay can run to five figures, and it is not covered by buildings insurance as a pre-existing defect.
A pre-purchase survey takes 60-90 minutes on site and produces a graded defect report with indicative repair costs. That report is negotiating leverage: either the price comes down or the seller fixes it before completion.
If the property has trees, a history of blockages, an extension, or clay drainage, a survey is not optional in any sensible reading of the risk.