This is the 17718th most frequent Russian word.
удобными
"Convenient" or "comfortable" (instrumental plural form).
In this sentence, 'удобными' is an instrumental plural form of the adjective 'удобный', describing the state or quality of the chairs.
Эти стулья оказались удобными для долгих собраний.
These chairs proved to be comfortable for long meetings.
Here, 'удобными' is again instrumental plural and describes the adjustments made to accommodate different groups.
Мы сделали их удобными для всех возрастных групп.
We made them suitable for all age groups.
The word 'удобными' is in instrumental plural form here, used in conjunction with the verb 'считать' to indicate the perception of the quality of the sofas.