What’s the difference between pathless layout routes (_segment) and pathless route groups ((segment))?

[Pyprohly]

Pathless layout routes (i.e., _segment) and pathless route groups (i.e., (segment)) seem to serve the same purpose, no? So what’s the point in having both conventions?

• foo/_segment/bar.tsx
• foo/(segment)/bar.tsx

(Similar discussion: #3837)