Because every member is sized for a specific force by the truss designer.

Fixing at an undesignated point introduces forces nobody sized for.

The resulting failure is not necessarily local.

It can affect the whole assembly.

The truss layout drawing states where connections may be made.

That drawing is structural documentation rather than a fitting guide.

Not without the truss designer's approval, and generally not at all.

Cutting a member removes capacity the design assumed.

Notching and drilling do the same to a lesser degree.

Services routed through trusses must use designed openings.

Damage discovered on site is referred back rather than repaired.

A damaged truss frequently has to be replaced rather than reinforced.

They restrain the truss against uplift and rotation.

Wind can lift a roof off a building that is not tied down.

Trusses can also roll if not restrained at the bearing.

Clips tie them to the wall plate or supporting structure.

It is a requirement in most markets rather than an option.

The specification comes from the wind loading for the particular site.

From the site's wind loading, not from the truss size.

Uplift forces vary enormously between locations and exposures.

The truss supplier's documentation states the required connection.

Local codes set the loading and the design method.

Coastal and cyclone-prone regions have substantially higher requirements.

A connection adequate in one region can be badly inadequate in another.

It is the safety issue that sits alongside every truss job.

Trusses are stable as a braced assembly and unstable individually.

Progressive collapse during erection injures people regularly.

Temporary bracing is installed as trusses are placed.

Permanent bracing is part of the design rather than an extra.

The truss supplier specifies both, and the specification is not optional.

In where they may be used more than in how they look.

The connection point is nominated by the truss designer.

Capacity has to suit the reaction the truss actually delivers.

Girder trusses carrying other trusses need substantially more capacity.

Standard joist hangers are frequently not suitable.

The truss documentation states what the connection has to achieve.

The truss designer specifies, and the installer follows the documentation.

Trusses are supplied as an engineered product.

The supporting structure and its connections are part of the design.

Site changes need referring back rather than resolving locally.

Requirements differ between markets and are covered by specific standards.

The truss supplier's documentation and the local code govern together.