Reset sequence to current maximum value of rows

Two tables have a sequence for the PK. Over time I manually entered the PK
numbers not being aware of applying DEFAULT to generate the next number.

I just tried to set one table's PK sequence to the current max(PK) value
using this expression from a stackexchange thread:
SELECT setval('<sequence_name>', <current_max_number>, true);  -- next value will be max(PK) + 1

Needing to add a new row to a table for a specific industry table (with 52
rows) I set the PK as DEFAULT in the INSERT INTO expression. To my surprise
and disappointment all 52 rows now have the company_name column as the newly
inserted name. Feh! I need to restore all the correct names for each PK.

There's an alternate expression in that SE thread that I didn't try:
ALTER SEQUENCE <sequence_name> RESTART WITH <next_number>;

I want to avoid this same situation when resetting the second table's PK
sequence number and would like to understand why the SELECT expression
changed all column values rather than adding a new row with its attributes.
And how to I reset sequences to ignore all current values while adding the
next higher value to the end when a new row is INSERTed.

TIA,

Rich