Book Flights or Not before Visa application

[mugfak]

I am British, my wife Non-EU. We live together in Finland where my wife works. We will be applying for the Family Visit Visa.
Due to the nature of my wife's work it is difficult to get exact dates for holiday in advance, meaning that we are not exactly sure when we want to travel to the UK.

The question I have is; do we NEED to buy the tickets for the flight before the visa has been approved?

I have read the guidelines urging people applying to not buy tickets before the visa, yet how do you prove intention to leave?
It is leaving me slightly baffled

Any guidance or relatable experience gratefully received!


EDIT
I have read on PDF from a UK university that it will suffice to have "evidence that they intend to leave the UK at the end of their visit (for example, a letter from their employer)". Has this been the case for others?

[ban.s]

I have read the guidelines urging people applying to not buy tickets before the visa, yet how do you prove intention to leave?
It is leaving me slightly baffled

Intention to leave is normally proven though evidences of ties with the home country / place of residence e.g. proof of employment, study, property, estate, finances, family commitment etc.

A return ticket can be cancelled, postponed etc; so mere possesion of a ticket doesn't necessarily mean intention to leave;
similarly a return ticket can easily be purchased with cash/card at any point, even during the UK visit - so lack of ticket doesn't mean that a genuine visitor would not be leaving.