In many jurisdictions, there is a general legal principle that people are entitled to trust certain kinds of professional opinion absent they have a particular reason not to. Someone who, e.g., build a balcony without hiring an engineer would have a duty to know what the structural requirements would be. If such a balcony collapsed because of bad design, the builder would be liable for breaching such duty. If, however, the builder were to hire a licensed engineer whose documented professional opinion was that it was safe, the duty to know about structural requirements, and the associated liability, could be transferred to that engineer (the exact circumstances where liability would transfer will vary by jurisdiction).
When a lawyer gives a professional opinion, the layer will often not only giving the client information, but will in many cases also be staking his status as a legal professional on the correctness thereof. A lawyer should not generally be willing to accept such risk without officially documenting all of the information upon which the opinion is based, and the amount of information needed to formulate an opinion that is 95% likely to be correct may be a tiny fraction of what would be needed to formulate an opinion that would be worth staking one's career.
Returning to the balcony example, if someone is considering building a balcony, they may want a rough idea of how much support it will need in order to decide whether the project is worth considering. If the the cost would be massively exceed the budget even given the most favorable ground conditions, there would be no point in ordering an examination of the ground for structural stability. If someone were to misinterpret a quick estimate of structural requirements suggesting that the product was viable, as though it were a reliable structural plan, and if they were to consequently build a balcony that collapsed as a result, the person who gave the estimate should not be liable since they hadn't given a professional opinion as to whether the ground quality was adequate for the suggested footings. Nonetheless, it would still be better for all concerned if the builder had hired a professional engineer.
Disclaimers are cheap. If adding a "No testing or analysis has been done to ensure the safety of this plan" disclaimer would have even a 0.001% chance of preventing someone from wrongfully assuming that the indicated structure could be safely built, that would be worth the cost of ink. Likewise IANAL, "this is not legal advice", etc.