Removing and installing mechanical fasteners (bolts and screws) is more complex than you might assume. It goes far beyond selecting the right size tool, and turning it in the correct direction.

A mechanic develops these skills gradually over decades- a master mechanic would often have no trouble removing a corroded bolt that a novice mechanic would destroy for example. Conversely, they can also install a bolt in a manner with less chance of failure down the line.

Some (but by no means all) of the issues involved include:

• How to brace your body to generate maximum torque in the right direction
• How to feel when a bolt is beginning to stretch while tightening, and stop before you break it (the torque stops increasing when a bolt stretches, and you can learn to feel this point)
• How to combine various tools to get leverage in tight spaces
• Which penetrating lubricants to use for various levels of corrosion on various materials, how to apply them, and how long to let them soak
• How much torque to apply when installing a fastener to minimize the chances of it loosening or failing
• Which anti-corrosion or anti-seizing coatings to apply for specific metals in specific use applications (such as high vibration, saltwater, high temps, etc.)
• Selecting the right type of socket (6 point, 12 point, impact, deep, shallow, etc. which have different strengths and weaknesses in different situations)
• How to select the right fastener diameter, and material for a given application (sometimes a vehicle- especially a boat used in salt water will have the wrong sort of fasteners installed by an incompetent person previously, and a competent mechanic will need to know what to replace them with)
• How to select and use bedding/sealing materials properly, for fasteners that must be waterproof or oilproof
• How to use a torque wrench, and ensure that it remains calibrated correctly

As someone who operates vehicles in remote areas (offshore boating, remote desert driving) skill in using hand tools has made potentially terrible situations much better and possibly saved my life many times...